Edge Sealing MDF

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Marineboy

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I've a feeling this topic has been aired before (it may even have been me who aired it) but I can't find the thread so am asking again.

I want to seal the edges of bog standard MDF for painting. I have tried everything - shellac sanding sealer, multiple coats of paint, wood filler worked in with a pallet knife etc, but no matter how many coats I apply, when I put on the paint coat the centre of the edge stays matt/fluffy while the borders look OK. Is there anything else that would seal the edge across its width? I understand that medite or similar is of a higher quality with more uniform density through its profile, and so this may be a better solution, but I'm not able to transport full sheets and the DIY sheds don't seem to stock it. So I'd really like to use the cheapie stuff if only I can get a good finish on the edges.
 
I use edging that’s designed for painting such as item 292252350358 on eBay. It can be easily applied by hand and paints perfectly every time. I’m lucky that I have an edgebander but have applied it by hand on occasions. I’ve used the stuff I’ve highlighted as well as others.
 
Thanks Deema. I am making small picture frames out of the MDF so a brush on sealant would be my preferred solution, but the edge banding looks a possibility. 22mm is too wide but I've googled 18mm and Screwfix do a Hafele version in that size which looks good.
 
I’ve tried most sealants, MSP paint works fairly well as does Zinsser 123 with shellac (red tin not blue) however they don’t in my opinion leave the surface as good as the face of the board without filling. With tape you get a perfect paint finish without any messing about.

I’d try to stick to Medite or premium brand MRMDF as the time and effort in painting standard / value MDF to get a really good finish far out weighs the slight cost difference. I might be wrong but I believe the density of a premium MDF / MRMDF is far higher than value MDF which makes for better fixings etc.
 
Thanks for the advice, I will definitely try the tape. I’ll use it with standard MDF first and see how that works out before considering the more expensive (and difficult for me to transport) medite.
 
A very simple solution which I have used mixed with polyurethane waterbourne varnish is an industrial filler called 'magnesium silicate'

You may know it as 'baby talc' :)

Depends if the edge is to be subjected to impact, but a mixture of dropping consistency ( no thicker or it will crack) overnight will dry to a sandable smooth edge. It does help to wipe the edge with pure waterbourne varnish or paint before applying to stop the mdf sucking the moisture out of the mix.

If you want a photo, I will see if I can dig out a scrap piece from somewhere.
 
Interesting, yes I'll definitely try that myself, so no need for photo but thanks anyway. As it is to finish picture frames there is no chance of impact damage.
 
I use MR MDF a lot and don’t do anything other than a thorough sanding > primer/undercoat > topcoat. MR MDF is way better than regular MDF on the edges.
 
I personally find sanding back waterbased varnish and baby talc subject to clogging even using stearated paper.

If you had a whole batch I guess you could do the same with cheap oilbased paint if you have some kicking around going spare, better seal of the edges and less prone to clogging.

I've used plaster of paris before now as a poor mans version of plasterboard jointing compound with a dash of PVA in it (and I had some spare after resetting the tiling in an iron fireplace for a customer) but jointing compound (Spackle in the USA) would also sand back to a lovely finish and takes paint well.

Alternatively I recently got put onto a powdered filler that is extremely good, will not pull out of even the smallest pores as you get in MDF, sands easily and takes paint extremely well:

https://www.screwfix.com/p/toupret-powd ... lsrc=aw.ds

It's BRILLIANT stuff - the 5 star rating is entirely justified - however I would be wary of using it under gloss paint.

Although the above options are water based they dry so quick there is little chance of the MDF edges swelling (assuming the mix isn't too sloppy), but if you are at all concerned about that, pound shop superglue, the really watery stuff in the plastic bottle pack of three (used to be 4) will seal it but you have to be quick applying it running a spatula over the edge right after running the bead of superglue to spread it out better - clamping the frame in a vice at an angle is advised. (also works for sealing plywood prior to planing a bevel to avoid tear out)

If the picture frames are going to be painted a dark glossy colour (that shows up EVERY blemish like a pinapple) and you need a glass smooth surface - you have options of either 2 part car body filler, a bit harder to sand and be careful to use a sanding block with a 90deg guide to keep it absolutely flat on the edge or you'll have issues with the outer edges and "rolling over" exposing the coarser MDF underneath, or you could try your hand at using Gesso which is the base used for gold leaf; the proper stuff has a hide glue base NOT water and as such won't promote swelling.

Also note that Halfords sell the epoxy body filler in large containers for about £24 compared to screwfix et al who sell only smaller tins for about £10.

If you want it a bit runnier, making it easier to apply in a thinner coat (with suitable spreader) buy a can of fibreglass resin at the same time as the two are compatible and mix it about 60% filler 40% resin - it'll go super gooey and liquid, almost pourable and will spread really well and evenly with less sanding required.

DON'T GO MAD WITH THE ACTIVATOR - you can get away with much less than the "1 pea sized activator to golf ball sized filler" commonly stated ratio - it takes longer to set, but not massively longer and gives you more working time so you can mix up more and less / no waste going off before you can apply it. I regularly use much less than that especially when the mix is runny as it mixes far better - a tip is NOT to mix up the resin & filler together first but add the activator to the resin, it'll dissolve really quickly into the resin, then mix the resin and filler for a good even mix so it all goes off properly - doing it the other way risks areas of "not gone off" filler with is then a right pig to deal with.

Warning - it WILL STINK - and the smell lingers so don't do it indoors - you might be able to contrive a line and some wire to hang them outside under cover for a day or so to let that dissipate.

Sand to at least 320 grit or even 400 and you'll have a finish that when painted with high gloss paint will be indestinguishable from the smooth face. Then make sure you "flat out" (wet sand) the paint using above mentioned guide block between 2nd and 3rd finish coat with 600 grit or you'll blow through and "roll over" the edges.... again - been there, got the t-shirt).

As you can see I've a bit of experience with this.

hth.
 
+1 for Isopon P40 (car filler) I use it on MDF that is to be painted and has been shaped on a router. I route the edge and then put a good smear on and then put it through the router again the next day. A couple of mist coats of filler primer and then a quick sand with 320 and then painted. Will post pick of a drawer front I did recently in the morning when #i find where i put my phone
 
Thanks for all those replies, I have plenty of options now! I paint my frames with spray satin, so surface defects don’t show so readily as gloss. I’d be interested to see the photo of your method Droogs.
 
I use Zinser BIN. It’s shellac based and does a good job although it’s still important to sand edges properly before and after application. Ive used a strong mix of dewaxed blonde shellac as well in the past but the Zinser stuff is more convenient (if spendy).
 
memzey":impqmv9z said:
I use Zinser BIN. It’s shellac based and does a good job although it’s still important to sand edges properly before and after application. Ive used a strong mix of dewaxed blonde shellac as well in the past but the Zinser stuff is more convenient (if spendy).

I recently made a batch of shellac, first time, to see if I could replicate Zinnser a bit cheaper and even a 2.5lb cut wasn't as good as the bought product, it seems to have a filler added (plus tint) that doesn't seem to be affected by thinning it out to a consistency similar to the 2.5lb cut make it flow better, so worth the money IMHO.

When thinned (methylated spirits) to about the consistency of single cream it flows much better and gives you more working time, not much more, but a bit more, to lay it off or use a roller without dragging - sponge not fluffy!

Not cheap though as stated - but shelf life seems to be at least 3 years (that's about how long I've had the first 5l tin I bought), give or take and used it just last week, so buying a larger tin isn't such a risk.
 
Hi Rafe. How’s you mate? The shellac didn’t work as well as the Zinser stuff to be fair (needed another coat iirc) but both definitely benefit from thorough sanding to get the best from them. Zinser wins on the sheer one and done convenience of it, once you’ve swallowed the price!
 
Droogs":1zypgz2c said:
+1 for Isopon P40 (car filler) I use it on MDF that is to be painted and has been shaped on a router. I route the edge and then put a good smear on and then put it through the router again the next day. A couple of mist coats of filler primer and then a quick sand with 320 and then painted. Will post pick of a drawer front I did recently in the morning when #i find where i put my phone

I'm pretty sure you meant P38 since P40 is the stuff with strands of glass in the mix.Similarly there is a mention in this thread of Halfords selling epoxy filler,when I'm fairly certain that the only fillers they sell are polyester based.Which is probably a good thing since polyester smells a bit and can annoy people,on the other hand epoxy can cause lifelong sensitisation and that really isn't a very nice prospect.

To get back to the topic of sealing MDF-I find sanding sealer excellent but with the proviso that not all of them are created equal.I rate Rustins but a couple of months ago I bought some from another source and it was ultra thin and a pale opaque white.I used it on MDF and the first coat just disappeared,after rubbing down the very fuzzy surface and applying a second coat the surface texture was better but the opacity was very noticeable.With Rustins or any of the quality brands I would expect the first coat to need a good de-nibbing with 180 Fre-cut or similar and then to be ready for finishing.
 
memzey":3q2pia5f said:
Hi Rafe. How’s you mate? The shellac didn’t work as well as the Zinser stuff to be fair (needed another coat iirc) but both definitely benefit from thorough sanding to get the best from them. Zinser wins on the sheer one and done convenience of it, once you’ve swallowed the price!

Hi Memzey - I'm good ty, the homemade shellac worked well enough for what I used it for recently, but didn't have the body of the zinsser when thinned to the same consistency and didn't save a great deal of money either to be honest. Paul Sellers notes that the only good reason for homemade vs manufactured is you know it's fresh, where it's been stored and has not much else in it such as additives.

The price of the zinnser is offputting but now I know it can be thinned by 30 - 50% with little effect on it's performance (or so I've noticed), if anything it's better, thinner coats equals less brushmarks and flashs off even faster) a 2.5 ltr tim goes that much further and lasts a darn sight longer than the homemade 6 months shelf life.

The advantage of being able to get 2 or three coats on in only a few hours pretty much regardless of low ambient temperature is also a big bonus compared to other primers, giving enough body for sanding back to give a base for a really good finish.

I made a small decorative box about 6" square over xmas to house a xmas tree bauble I made that's going to be shipped to America - (at least it will be when I've made the other one) 3, 20% thinned coats of zinnser bin, flatted back, another even thinner coat, sanded again and then spraypainted and it's like glass - started off as pretty rough cheap plywood just 24 hours earlier; find it hard to get that finish in the same timeframe with anything else.
 
For Marineboy
Here's some picks of the drawer fronts for my new router table.


1 Put the MDF through the table as normal and then smear a thin layer of Isopon (car filler) on and once dry put through again with out changing settings on the router https://1drv.ms/u/s!An_F5-xpP08UnkPhdLQynkq41qQ-
It can be very messy if you don't have dust extraction.

2 Then a couple of thin (mist coats) of filler primer https://1drv.ms/u/s!An_F5-xpP08Uo1FIIka-Nz2hxmbk

3 Rub it down starting at 240 and then 320 https://1drv.ms/u/s!An_F5-xpP08Uo1lrKr8vM5_gAhFc

4 Apply base colour coat, first a thin mist and then 2 coats https://1drv.ms/u/s!An_F5-xpP08Uo1D4wg0xs-cwgXtq

5 Then apply final laquer finish 2 thin coats - more of heavy use is envisioned https://1drv.ms/u/s!An_F5-xpP08UpAuP9EAvFc-_KrrW

and Bobs your mothers sisters lodger as it were
 

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