Spraying Paint

[SimonStevensCanes]

I've been stuck on the painting phase of my pantry build for a few weeks now and think it's time to ask some experts. For context, I'm currently spraying flat (no detail/moulding) MRMDF drawer fronts/doors.

Problems that I'm encountering

1. After spraying I have occasionally found long streaks (as wide as a 5p piece, maybe 15cm in length) in the finish. I suspect this is oil finding its way in.
2. After spraying I sometimes find what I can only describe as "scabs" in the finish. They're smallish (5p piece) bubbly raised areas. Only thing I can think is that it's oil again, but it's a very different result from the above.
3. During spraying I struggle between getting enough paint on, and pooling/dripping. I suspect that this is a combination of (over?) thinning the paint, and inexperience.
4. After spraying I de-nib with 600 grit. When I focus on problem areas (see pooling/dripping above) I invariably end up overdoing it and taking surrounding areas back to wood (MRMDF). Obviously "be more careful" is a solution, but does anybody have any techniques to help with this?
5. After drying, the paint stays (very) slightly tacky. I can leave a fingerprint on it, which will go away within seconds. If I leave a painted item on something, the paint may stick to it and tear off when moved. Leaving two painted things touching each other will bond them fairly tightly, and will definitely result in paint being torn off. This situation doesn't appear to get much better with time. I have some items that have been drying for a week or more, that still have this issue.

Possible issues

1. I'm spraying water based acrylic gloss that I thin with water.
2. I'm using a compressor and cheap amazon spray gun. Compressor has a pressure regulator which combined with a little twiddly knob on the gun lets me tweak the pressure adequately (I think). I don't have any concerns with the gun, it seems up to the job.
3. I have an inline water filter/trap on the gun. This is looking a bit worse for wear, may need replacing.
4. I'm spraying outdoors. Bugs are a pita, but not much I can do about that other than move stuff indoors for drying.
5. For drying large batches I stack pieces together with craft paper to keep the painted surfaces from touching. Could this be inhibiting further drying/curing?

Appreciate any advise that you can give .

 

[AES]

I'm no expert but have had some success - but ALWAYS after some trial and error tests first (that's every time I change paint type). Some comments re your above, but I repeat, I'm NOT an expert:

1. Yup, it sounds as though your moisture/oil trap may be clogged. On some you can take them out to clean the filter, on some you can even replace just the filter, on some you just have to buy a new one. Check yours;

2. Yup, it sounds as though you MAY be over-thinning your paint. The (long-term-ish) fix is to buy a Viscosity Cup, quite cheap. The idea is you have a calibrated hole in the cup (a sort of funnel shape) and when doing the above trial and error with a new-to-you paint, you keep thinning until you get the finish you want. You then fill the vis cup with the same mix and time how long it takes for the mix to run out. Note that time and the next time you use that particular paint you can make a mix until it takes the same time to run out of the vis cup. You can then be pretty sure that that mix will produce the same results as before. It just so happens I use a Ford No. 1 vis cup (that's what the guy in the - specialist - shop had in stock), but what you buy isn't really important. It's just a good help if you use lots of different paint types;

3. Does your compressor/spray line/spray gun have a pressure regulator? If so try turning the pressure WAY down. You MAY be using too high a pressure, and/or be having the nozzle too close to the job. Either/both these can cause the sort of "bubbling/blistering" effect you mention above. So can have even tiny traces of "grease" (even just finger prints) on the job. Especially important since you're spraying a water-based paint (oil & water do NOT mix)!;

4. Re the paint remaining a bit "sticky" that MAY be over-thinning again, MAYBE you not mixing the paint and thinner VERY thoroughly, or MAYBE some residue (grease again?) left on the job. Also with a water-based paint the pigments are often pretty dense so ALWAYS filter the paint (after stirring and before thinning). Then filter again, after you've got the mix you want. Old tights are excellent for that;

5. IF the job allows, arrange some pattern of nails or something so that when you lay the just-sprayed job down to dry, just a few tiny points are touching the freshly painted surface;

6. Yup, drying outside is a real PITA, especially if it's warm enough to attract mossies, etc. I don't really have an answer for that except to try and rig up a "drying tent" of some sort. Use old bed sheets, bath towels, curtains, etc, rather than plastic sheets if you can (after first use, paint dries hard on plastic sheets then flakes off - all over the job - next time you use them). Do NOT ask me how I know that!

Again, ALL the above are just "maybes" and with luck a proper painter will be along to help you much more.

Meantime if you've haven't already, I recommend you look up the YouTube channel "Ten Minute Workshop" by Peter Millard (member here). He runs/ran a business doing all sorts of built-in cupboards, wardrobes, etc, and used mainly MDF and water-based paints, all sprayed. He did a whole vid series on spraying, well worth a watch.

Good luck, HTH

 

[SimonStevensCanes]

thanks AES, some useful information in there. To answer some of the things you've raised (edited the OP to reflect also)...

2. Yup, it sounds as though you MAY be over-thinning your paint. The (long-term-ish) fix is to buy a Viscosity Cup, quite cheap. The idea is you have a calibrated hole in the cup (a sort of funnel shape) and when doing the above trial and error with a new-to-you paint, you keep thinning until you get the finish you want. You then fill the vis cup with the same mix and time how long it takes for the mix to run out. Note that time and the next time you use that particular paint you can make a mix until it takes the same time to run out of the vis cup. You can then be pretty sure that that mix will produce the same results as before. It just so happens I use a Ford No. 1 vis cup (that's what the guy in the - specialist - shop had in stock), but what you buy isn't really important. It's just a good help if you use lots of different paint types;

I've been eyeballing it in the cup, which felt relatively successful. I guess maybe I need some more experience before I can do that so I'll go back to my viscosity cup. I assume from a paint application perspective, the thicker the better, and then it's just a question of what I can get the gun to spray well.

3. Does your compressor/spray line/spray gun have a pressure regulator. If so try turning the pressure WAY down. You MAY be using too high a pressure, and/or be having the nozzle too close to the job. Either/both these can cause the sort of "bubbling/blistering" effect you mention above. So can have even tiny traces of "grease" (even just finger prints) on the job. Especially important since you're spraying a water-based paint (oil & water do NOT mix)!;

My compressor has a pressure regulator and the gun has a dial I can tweak (although it's rudimentary with no readout). The combination seems to get me a decent spray pattern.

I'm fairly confident that the nozzle being too close to the job isn't a problem. I have on occasion intentionally moved the nozzle close to the job in order to pool paint in an area that needs some filling, and this hasn't resulted in bubbling/blistering.

Finger prints is a thought though, the blistering is roughly fingerprint shaped. I guess I need to give things a wipe with some acetone(?) before spraying.

Traces of grease... am I relying on the water trap to catch those also? What else could I do to prevent this?

with a water-based paint the pigments are often pretty dense so ALWAYS filter the paint

This is interesting, I'd like to hear more on what "dense pigments" means/results in.

I had stopped bothering with filtering as the filters I use lose all structural integrity after just looking at them. Any suggestions for a good supplier would be welcome.

 

[AES]

First, a bit about denibbing which I forgot 1st time:

It sounds as though either your finished paint coat is a bit thin, and/or you're rubbing too hard. 600 sounds about right, but perhaps try 1000 if the problem persists? May I also suggest ALWAYS use a sanding block (scrap of wood, ali extrusion, cork, w.h.y, but make sure the under surface is FLAT (!)) and make sure the paper does not extend beyond/behind the front and back edges of the block and always have enough wrapped around the sides of the block so that you can grip the paper without it slipping around the block a bit. And for me anyway, NEVER be tempted to "just give it a touch up with just fingers" - always a flat block. For me anyway, fingers are a GUARANTEE that I'll accidentally rub thin spots into the paint.

Re viscosity, I wouldn't say "the thicker the better", I'd say "as thick as you can spray it without getting runs". A very rough guide (but I found helpful before I bough a vis cup) was "about the same viscosity/flow rate as milk".

And as always "several thin coats are better than one thick coat".

The pigment in acrylics is definitely thick and heavy (so sinks to the bottom of the tin - on your shelf, or in the shop - when stored for a while. For that reason I always "attack" the tin before opening by shaking vigorously before opening, then when open, stir with a clean stick, then with a "paddle" on a electric drill. After that you need to wait a little for the aeration to disperse.

I then ALWAYS filter it (tights) before thinning (helps with the air bubbless too), then filter again (really, again!) after I've got the viscosity I need. Personally I would never ever stop filtering before spraying (several times, as above).

Yes, I find acetone a good de-greaser (but not on cellulose paints!!!!), and a tack cloth and nail varnish remover also useful. I also always use disposable nitrile gloves too (for my hands!!!!!) - "cleanliness is next to something or other" (and all that)! Make sure your rags are clean & dry, or use the jumbo kitchen rolls from the supermarket.

Not sure what you mean about filter suppliers. If you mean for the gun/compressor, I guess it depends on the manufacturer. If for general paint use, I use ladies tights. Also (don't know if they're available in UK) some coffee machines here filter the coffee through a "paper pyramid" which you buy in packets in the supermarket. Only works for really thin mixes/small amounts (otherwise it takes a million years to get through) but MAY help?

I'd also recommend you find a friendly local professional paint/car spray shop. You'll get all sorts of filters, measuring cups, tack cloths, vis cups, etc, etc.

HTH

 

[novocaine]

I've been eyeballing it in the cup, which felt relatively successful. I guess maybe I need some more experience before I can do that so I'll go back to my viscosity cup. I assume from a paint application perspective, the thicker the better, and then it's just a question of what I can get the gun to spray well.

no. thicker is not better. correct viscosity is very important. this explains why you are getting runs, blisters and such.
use your cup. even the pros still do it, you simply can't eyeball viscosity. I use a ford #4 (which I reckon is what AES uses too as I've never heard of a ford #1 sorry AES old bean) , a good target is 90-120 centipoise which is around 30 seconds run time with a #4. get that right and most issues go away.

filtering is a very important step and really shouldn't be ignored. contaminates can really screw up your pattern and result in some of what you are describing. everything as, do as AES says. I use these because I only wear tights on the weekend so don't have enough to waste on paint straining.

https://www.amazon.co.uk/Fast-Mover...ocphy=1006901&hvtargid=pla-697125957733&psc=1

 

[AES]

I DEMAND to see a pic of you in your weekend tights novocaine (as per the site rule, "no pic = it didn't happen")!

Thanks for clearing the vis cup confusion mate - Yes, a No. 4, and Yes 30 secs for the stuff I use.

Cheers

 

[novocaine]

I DEMAND to see a pic of you in your weekend tights novocaine (as per the site rule, "no pic = it didn't happen")!

Some things should remain unseen. this is one of them.

 

[Ollie78]

Pictures will help.

I can help with you problem of over sanding on drips and runs.
Do not sand it, use a very sharp cabinet scraper or carbide blade to scrape back the high spots until its flat. (You can buy tools for this specific job)
Only then give a quick pass with the de nibbing pad.

Do not stack them together or touching anything. They need air all around them, what is happening is called blocking.

You will learn the right speed with experience, there is no substitute for practice.

Ollie

 

[AES]

Some things should remain unseen. this is one of them.

I'll accept that excuse novocaine - just this once though

 

[pe2dave]

1. I'm using a compressor and cheap amazon spray gun. Compressor has a pressure regulator which combined with a little twiddly knob on the gun lets me tweak the pressure adequately (I think). I don't have any concerns with the gun, it seems up to the job.

No one mentions the gun itself? A possible source of problems?

 

[doctor Bob]

some of those denibbing grits being talked about are ridiculous.......................

unless it's burnished high gloss but I get the feeling it isn't.

 

[novocaine]

No one mentions the gun itself? A possible source of problems?

unlikely to be causing the issues seen, normally you'd get a poor finish across the entire thing if it was the gun. blockages are more likely to be causing localised issues, hence, filter. Oils would be localised too.

some of those denibbing grits being talked about are ridiculous.......................

unless it's burnished high gloss but I get the feeling it isn't.

I don't see the problem with 600grit for denibing Bob, I wouldn't go any higher but 400-600 is about right, 600 will just take longer to knock off the tips.

 

[doctor Bob]

I'd say 600g is too high, someone mention 1000g. Having said that my spray shop is a production line and speed is important but we rarely go below 320g.
I suppose it depends on the quality of your base coats.

 

[AES]

It was me who mentioned 1,000 grit doctor Bob (because the OP said he was rubbing through to bare MDF). So I suggested either his coat/s were too thin, he was rubbing too hard/without a flat under-surface block, OR he should try going down to 1,000. That grit (3M W&D) has worked well for me.

We all have our likes and dislikes, and as I said in my own original reply, I'm NO expert AND I certainly don't run a production line!

Basically though, I feel that if the paint is properly thinned and filtered, there is little very little in the way of "real sanding" required to de-nib. Personally, I would never use a scraper as someone else mentioned above, but at the end of the day, I feel that whatever works for the individual is "right".

The OP should try all the suggestions above and find out what works for him - IMO.

 

[SimonStevensCanes]

no. thicker is not better. correct viscosity is very important. this explains why you are getting runs, blisters and such.
use your cup. even the pros still do it, you simply can't eyeball viscosity. I use a ford #4 (which I reckon is what AES uses too as I've never heard of a ford #1 sorry AES old bean) , a good target is 90-120 centipoise which is around 30 seconds run time with a #4. get that right and most issues go away.

Ok, got it. I have been trying to cut corners to overcome the additional effort required with spraying. Looks like those corners are necessary and I just need to suck it up.

I have a viscosity cup, but I didn't realise they come in different sizes(?). I'm guessing the only way to be sure that what I have is comparable is to replace it.

Do not sand it, use a very sharp cabinet scraper or carbide blade to scrape back the high spots until its flat. (You can buy tools for this specific job)
Only then give a quick pass with the de nibbing pad.

Perfect, I knew that there must be a better tool for dealing with high spots, I just couldn't think what it would be. Cabinet scraper makes perfect sense . Might a razor blade also be suitable?

Do not stack them together or touching anything. They need air all around them, what is happening is called blocking.

blocking... nomenclature is so important when learning something new. It's really difficult to google for things that you don't have the right terminology for. I guess that rules out knocking out a big batch when weather permits and stacking them up when touch dry. Painting is a pita in small workshop .



Thank you all, I have read and absorbed all of your wonderful advice, even if I haven't acknowledged it. Feeling a lot more confident now.

 

[novocaine]

you cup is most like a ford #4. you can measure the hole to find out. 4.12mm for a #4. not very interestingly, I was looking for something else and found that a #1 does exist, but its for far more runny liquids than paint with a 1.9mm orifice.

 

[AES]

@Snettymakes: You wrote QUOTE: "Ok, got it. I have been trying to cut corners to overcome the additional effort required with spraying. Looks like those corners are necessary and I just need to suck it up." UNQUOTE:

Yup, I'm afraid it's exactly that in my experience - you do need to "fiddle all the little corners" - it's what the professional (aircraft etc painters) that I've seen working their jobs always do (if they're any good).

I sometimes think that if it wasn't for the fact that - WHEN "all the corners are all properly handled" - you end up with a superior finish, it's hardly worth spraying at all in a non-professional/non-production situation - what with all the paint mix prep, job prep and masking, then all the cleaning up afterwards it sometimes hardly seems worth it.

Which is why (high cost be-damned) I buy rattle cans for the small bits & pieces like toys etc I sometimes make - unless I can convince SWMBO to snap into action that is (she LIKES painting, but then she always leaves me with the cleaning up afterwards anyway)!

 

[sometimewoodworker]

After spraying I sometimes find what I can only describe as "scabs" in the finish. They're smallish (5p piece) bubbly raised areas. Only thing I can think is that it's oil again, but it's a very different result from

After drying, the paint stays (very) slightly tacky. I can leave a fingerprint on it, which will go away within seconds. If I leave a painted item on something, the paint may stick to it and tear off when moved. Leaving two painted things touching each other will bond them fairly tightly, and will definitely result in paint being torn off. This situation doesn't appear to get much better with time. I have some items that have been drying for a week or more, that still have this issue.

the above.

First section That is a contaminant under the paint.
I've never had that, possibly as my first layer is always shellac, and I never touch the items without gloves, I have 2 filters on the airline, one near the compressor one on the line I plug my spray guns into.

For the second problem never allow uncured painted items to touch anything other than painters primids, until they are cured, never stack them until they are cured hard. If your surfaces are sticky after a week you have significantly messed something up. You could have old paint, bad water, too much water, too cold, too humid, etc. Never add more than 10% preferably not over 5%. If you can’t spray like that then your spray tip is probably too small. What size tip and needle are you using?

Brush a test piece with undiluted finish. It should be dry in an hour or so, hard enough to handle in a couple of hours. If it isn’t then bad paint, too cold, too humid are the answers.

I can put on 3 to 5 coats in a day depending on when I start with that kind of paint, with shellac I haven’t counted as it’s sanding hard in an hour or less, so maybe 8+ Depending on the area being sprayed, if I’m not sanding then maybe 4~8+ coats per hour or so.


The spray gun is not the problem, I have one gun that cost $600 others that cost about ฿600 (£13) I can get a similar finish with both. The cheap gun only really works with the container on the gun, the expensive one can have a pressure pot attached so can spray 5 litres or more before I need to add more finish.

 

[johnnyb]

I'll try and give my tuppence. a compressor gun will struggle to spray water based unless it's really thin(these guns will spray thinners based stuff with the right tip)
over thinned paint won't hold out ie it will slump to the bottom of surfaces.
problems like you mentioned are best ignored when they happen once or twice. if it happens on lots of pieces give it some consideration. flatting water based is best with 240 to 320. nibs are inevitable after the first coat on wood. if you just rub the flat and avoid the corners you'll be fine. After the first coat much less nibbing is needed( if any)
try and spray bin sealer as that is the easiest stuff to spray.thin and clean with meths.
I'll be honest no off the peg paints are as good as say ankerstuy or sayerlack many fall very short of ideal tbh.
the best I've used are from bedec from toolstation aqua advance or msp.
really they require a min 1.8 possibly a 2.2 or 2.4( they usually come with a 1.2 or 1.4)
ankerstuy sets in 2 hrs and can be stacked next day with minimal sticking.
bedec can be a bit sticky for a day or 2.

 

[RobinBHM]

I assume from a paint application perspective, the thicker the better

The starting point is to thin the paint enough for your gun to atomise properly. Paints with high solid content and high viscosity won't get atomised by a 1.0mm tip

The best way is to do some sprayouts to get a feel, you need to aim for a nice oval fan pattern which is laying wet....if it's spattering and coming out dry the paints too thick for the nozzle.

Once you have a nice even wet coat, if it's orange peel, you need to thin, if it runs quickly it's too thin.


One good tip: make sure your paint is warm - keep it in the house so it's 20 degs
And you can't paint really cold surfaces.
Obviously you need warmish air for good drying.

Waterborne paints are tricky - I find spraying top coats there is a fine line between too heavy a coat and too light. Too heavy: runs too light: pin holes.