Spraying Paint

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RobinBHM

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Denibbing and rubbing back paint is best done with a 320g or 400g abrasive - but you need foam backed pads like siasoft.
 

sometimewoodworker

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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)
I disagree. I have sprayed and continue to spray water based finishes

I don’t do airless or turbine spraying.
Apart from shellac I do very little to no solvent thinned spraying, I don’t like having to use organic solvent cartridges on my dust mask. Dust filters are enough for WB and shellac.

I add a little water to some finishes not to all, I have a Ford cup but seldom use it. The finish is similar to single cream in consistency not milk. I would not call that really thin.
The water based finish I spray the most is a polyurethane I use at a clear coat over finishes. The thing I need to keep on top of is dried finish building on the gun, this happens after spraying for around half an hour, it takes a few seconds to a minute to clear. I’ve mostly been using a 1.8 tip, bigger may be better but you have to get them with the gun & I’ve only just recently got one with 2 & 2.5 so can’t comment.

I have sprayed a little latex house paint for some smaller trim items.
 

johnnyb

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I use air assisted airless and don't thin at all. the finish is excellent. so basically we are thinning solely to go through the gun. water based is already a thin coating as its not got much body. the best finish comes from using a filler primer like hydrolux by ankerstuy. it just gives body to the finish. it's thick but still sprays through a normal tip.(0.011 but that's airless sizing) I find also to get great finishes I need to have good wet film thickness otherwise it won't level.
I do occasionally spray with a hvlp turbine but it introduces issues like overthinning, bad holdout , incorrect wet film and slow spraying meaning overspray issues.
I use a 1.8 or 2.2 tip. bedec aqua advance is quite liquid.
 

BillK

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Is it a brand new gun? Needs stripping down and cleaning in gunwash first usually, internals can be oil coated.

Panel-wipe the surface first, any oils/fingerprints etc need getting rid of. I've only once sprayed MRMDF but it was hard to degrease, I got resistant spots even after panelwiping. Doing it 2-3 times with time gaps between might've been better.

Re the fingerprints after spraying/initial dry; I haven't sprayed anything waterbase for ages but the right coat thickness and warmth & especially air movement helped drying. After the initial touch-dry part it's still going to want to carry on drying, stacking sheets say would greatly slow that part of the drying curve.

If sanding out sags or suicidal insect squadrons, use sanding blocks and not bare hands. You can make blocks or sticks of course, one little thing is softening the edges/corners makes them less likely to leave tramlines.
 

MikeJhn

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To the OP, have a read of this article: HVLP investigation, I would also recommend using a primer filler surfacer on the MDF first as someone did mention above.

To the poster that says he has problems with overspray with a HVLP, there is something wrong with the way you are using the equipment, the thing about HVLP is the overspray is practically zero with proper technique, also the warmth of the air coming through the gun from the turbine accelerates the drying process, mitigating the bug problem as they can be blown off almost immediately they land.
 
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johnnyb

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I have problems with overspray on hvlp because using water based its to slow. OK on say a 4 or 5 inch newel. but murder on say a door. one statement hints that the paint is nearly dry because of the warm dry air. this tells you overspray from the last bit will make the first bit rough. ie its very slow putting a coat on.
 

Ollie78

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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.

I was only saying about the scraper to deal with runs, pooling and sags specifically, not to de nib the lot. Festool and Mirka both sell carbide tools just for this purpose but any decent flat sharp thing will do, a 2 inch chisel in a pinch even.
The reason this method is good is because it starts taking of the run from the highest spot with no effect on the surrounding paint, sort of planing it down in micro scale.
The problem with any abrasive in this case ( particularly a soft de nibbing pad ) is the difficulty in avoiding the thinner paint around the run which then gets worn through.

Ps. Once you have gone to air assisted airless you don`t really have to care about thinning paint anymore and all that fiddling with cups and timing etc.

Ollie
 

MikeJhn

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I have problems with overspray on hvlp because using water based its to slow. OK on say a 4 or 5 inch newel. but murder on say a door. one statement hints that the paint is nearly dry because of the warm dry air. this tells you overspray from the last bit will make the first bit rough. ie its very slow putting a coat on.
When spraying doors I start at one end and immediately put on another coat and then another if needed, its much quicker than
Wardroabs - 1 of 1.jpeg
compressor spraying.
 

JonOuk

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I’d consider this to be my area of expertise…
Water-bourn on mdf isn’t the best as it makes the fibres puff up. And really you need a primer first, this will fill the edges better and be easier to sand than coat on coat (top coat) that your trying to do, this is part of the reason it feels semi-cured.

The surface needs ventilation to go off, stacking will prolong/prevent proper drying…
I stack them staggered against a wall, like leaning dominoes, with space in between each.

10% thinners is a good ratio to aim for, depending on the paints viscosity more can be added if needed.

you want to have 30/40 psi of air at the gun, less will cause poor atomisation and too much will cause excessive overspray, dryness and waste paint to the air.

Ive got a £25 gun, works fine. Make sure to over lap by 50% and keep it consistently the same distance above the piece by you moving your whole body, not swinging the gun with your arm as you’ll get uneven coating
 

JonOuk

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Oh and mdf is quite forgiving when rubbing trough the paint, as long as you don’t concentrate on one area and make a dip,
 

Snettymakes

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You haven't been using silicon products near your board's by any chance

Nope, I saw another thread about that, so I'm aware that it's a dangerous thing to have around painting/finishing.

I’d consider this to be my area of expertise…
Water-bourn on mdf isn’t the best as it makes the fibres puff up. And really you need a primer first, this will fill the edges better and be easier to sand than coat on coat (top coat) that your trying to do, this is part of the reason it feels semi-cured.

The surface needs ventilation to go off, stacking will prolong/prevent proper drying…
I stack them staggered against a wall, like leaning dominoes, with space in between each.

10% thinners is a good ratio to aim for, depending on the paints viscosity more can be added if needed.

you want to have 30/40 psi of air at the gun, less will cause poor atomisation and too much will cause excessive overspray, dryness and waste paint to the air.

Ive got a £25 gun, works fine. Make sure to over lap by 50% and keep it consistently the same distance above the piece by you moving your whole body, not swinging the gun with your arm as you’ll get uneven coating

I guess that my original post didn't say that I used a primer, I did. I did not denib (or anything else) the primed finish though, in line with comments/advice I've seen elsewhere.

I'd also be interested in your thoughts on "how much" to prime. I've always figured that getting a coat of primer on is enough, it doesn't need to be completely opaque and "perfect".

I used MRMDF as I'd heard that it is drastically different to MDF for water based products, but in all honesty I'm not sure I'd say it's "drastically" better, just a bit. Certainly the "end grain" is still problematic. Possibly I should have put heavy coats of primer on the end grain in particular.
 

RobinBHM

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I used MRMDF as I'd heard that it is drastically different to MDF for water based products

That depends on the brand.

Medite MR is better than kronospan or caberboard.
Finsa hydofugo is reckoned to be better still.

The green all the way through stuff is not medite - that is brownish surfaces with green core
 

sometimewoodworker

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I'd also be interested in your thoughts on "how much" to prime. I've always figured that getting a coat of primer on is enough, it doesn't need to be completely opaque and "perfect".
That depends on your topcoat, if the topcoat gives a solid colour with 2 coats the it’s OK and good enough, if it doesn’t then you need a flat colour with the primer however many coats that takes. That is my approach and opinion.
YMMV
 

Ollie78

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That depends on the brand.

Medite MR is better than kronospan or caberboard.
Finsa hydofugo is reckoned to be better still.

The green all the way through stuff is not medite - that is brownish surfaces with green core

Its funny but I kind of have the opposite view, I like the Kronospan best as it seems denser and more resin heavy. I have found it makes a better edge and seems to take a moulding for handle recesses and stuff better. I found the Medite to be "fluffier" in general.

Not heard of the Finsa stuff you mention but might try it out.

Ollie
 

paintman247

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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 🙏🏼.
Spots sound like oil from compressor, your not sanding your polishing with 600s to get lumps out, scrape first light sand with p400, topcoat is too thin, warm it up first in a bucket of hot water
 

RobinBHM

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Its funny but I kind of have the opposite view, I like the Kronospan best as it seems denser and more resin heavy. I have found it makes a better edge and seems to take a moulding for handle recesses and stuff better. I found the Medite to be "fluffier" in general.

Not heard of the Finsa stuff you mention but might try it out.

Ollie
Thanks for that - I shall have another look at Kronospan, it's possible I got that wrong.

I have found that better quality boards come from sheet material suppliers rather than local timber merchants.....which is also true for veneered mdf as there's a contract grade that they buy which is lower quality veneer.
 

Joshjosh

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I agree with the finsa comments, I'm paying more for finsa than medite, but the edges take allot less work to be paint ready and there's allot less fibre raising with wb primer. Enough that I can usually get away with 1 primer coat with the finsa. Only problem at the moment is they appear to be slowing down production and therefore pushing up the price
 
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