Thread: How to paint plywood

Depending upon the quality of paint finish you're after, you could try a couple of alternatives. If you're set up for spraying, swap to a pre-cat white pigmented undercoat - if the ply is rough (say C/D face), you'll need to put at least 3 coats on, cutting heavily between each. When it looks like it's choked, spray on a coat of grey pigmented undercoat (just mix up a batch of the white with a touch of black spirit stain or universal tint) and lightly sand this - high spots and imperfections will be mercilessly revealed.

Put a small amount of the undercoat aside exposed to the air and let it thicken til it's the same consistency as toothpaste - use this to fill the pinholes, divets etc etc. When dry, knock off, cut the whole surface again and spray another coat on. The advantage of using pre-cat is the speed of drying, sanding and re-coating - much quicker than water-based.

Don't put any topcoats on anything til the undercoating is giving you the surface quality you want - topcoats are only there to provide colour, not body or finish.

The other method which works really well on C/D grade is to use gesso as your undercoat - your local art supplier will have it. If you need a lot, check prices online. You can spray it on - brushing is just as easy. You'll probably need a couple of coats - and again, get your undercoating right and the topcoat will be fine.

And I should have mentioned that using water-based product on manufactured boards is, well, always a bit of a problem - grain raising, possible deformation... if you HAVE to use water-based, put down a sealer coat of say Zinnsser SealCoat - its a dewaxed shellac you should be able to order from your local paint shop. When using shellac under water-based finishes, be ABSOLUTELY certain its a dewaxed product. Or at least states that its suitable for use under water-based finishes.

Wow!! Many thanks for this information KD401- this is a real eye opener

Looks like a lot of experimenting coming up.

swap to a pre-cat white pigmented undercoat - if the ply is rough (say C/D face)

I have to plead ignorant, being new to spray painting. What is pre-cat, and what is C/D ?

Don't put any topcoats on anything til the undercoating is giving you the surface quality you want - topcoats are only there to provide colour, not body or finish.

thanks for this valuable advice - I have never read this anywhere before. Make all my efforts with the undercoat worthwhile

Originally Posted by JillB

and what is C/D ?

C/D is the grade of the ply.

I had a 1800 x 1200 sheet of 7mm ply to be painted, for use as a backboard for a large toy shelf, so I tried two methods, based on advice from KJ401. Both use water based acrylic 3 in 1 primer - sealer- undercoat.

Side A is the best side, side B is fairly rough

Side A:
Wet the board, sanded with 180 grit and then used Timbermate to fill the obvious cracks and grain and knots. Then sanded with 180 and 320 grit paper, before spray painting. Two long thin cracks showed up that I had not seen previously, but that was the only problem. It was quite a smooth finish.

Side B:
Sanded this fairly rough side with 180 sandpaper, then quickly slapped on some Wattyl Sanding Sealer I had left over from a previous project, let it dry for 2 hours, and then sanded with 180 and 320 grit paper before spray painting. It turned out to be a fairly rough finish, as expected from my rough sanding effort, with a fair bit of grain showing in places where it was really rough, but not as a result of raised grain from the water based paint, but from lack of thorough sanding. It would have been better if I had a) done a much better sanding job and b) applied some filler across the whole side to fill in the rough bits before painting.

So, not sure what I have achieved with this little experiment, as I was not comparing like with like - Side A was quite smooth compared to Side B, but certainly the sanding sealer prevented the raising of the grain by the WB paint on side B.

Does anyone know of a good filler that can be applied in a slurry form?

Pre-cat is pre-catalysed lacquer. Single pack - you don't have to add hardeners etc - just thin it as required with the correct thinner. Depending on where you are, you might/ should/ hopefully be able to access it via Wattyl, Miratone or Pylon Chemicals or someone who acts as their reseller. A local spray paint supplier (for automotive finishes) would probably be able to help as well

Pre-cats are very forgiving - use a pretty big needle and cap set-up on your gun - at least a 1.8mm tip if not 2mm. If you're likely to do a bit of colour work over time, it's worth buying one of those dreadful cheapo spray guns just for undercoating. Also worth grabbing some gunwash - cheap recycled thinners; you'll use a fair bit cleaning up.

Use a mask with the appropriate cartridges (which you should be using with water-based finishes if you're spraying them - they might be better for the environment, but they're not much safer than pre-cats re OH&S) and because they flash off so quickly, they're fine to use outdoors (just not under your neighbours windows or beside their car...)

Thanks for this information, KJ401, much appreciated. Yes, I always wear the appropriate mask for sanding and for spraying, esp with water based paints - the risks are not worth taking !