How to rectify tannin staining through paint?

Question

I am painting some wooden internal window sills and skirting boards, and a yellow-brown stain is showing through the paint. From the answers to What are the brown stains showing through my white paint? and Primed wood and painted with white paint but looks yellow. Why and how to fix?, it looks like it is probably tannin leaching out of the wood and through the paint. I'd like to know how best to fix the problem, ideally without having to sand back to the bare wood and start again.

I'm not sure what type of wood it is, but it was a reddish colour, and previously covered with a dark red-coloured varnish. The varnish has been scraped off, then sanded down and primed with white 'Valspar Wood Primer & Undercoat'. The stain was quickly visible through the primer, so a day or so later, a second layer of primer was applied before painting with white 'Dulux Quick Dry Satinwood for Wood and Metal'. While this initially looked OK, the stain is now starting to show through again.

The answers to the questions linked above talked about using primers to stop staining in the first place, but didn't really discuss how to make good stains that were coming through. After all the painting I've done so far, I'm not keen to sand it back to bare wood and start again with a different primer. Is there anything I can apply on top of the satinwood paint to stop the tannin coming out before a final coat? Recommendations for products I can get in the UK would be welcome.

Answer 1

I use a shellac based primer for things such as this. It has better hiding ability then just paint or regular primer.

Give it at least two coats or more and see if it hides the problem and then you can apply the final paint. It goes on very thin and appears to not be hiding the stain but don’t let that fool you it is creating a impenetrable barrier over the stain.

For added insurance you could give it a coat of a good bonding primer and then do your final painting.

Answer 2

You may have wondered with all these low odor paints available why would anyone use a smelly paint? One answer is to find a paint that will not absorb the stain.

You don't need to sand back to wood, just scuff sand to dull the sheen so that a fresh coat of different paint will stick. however if there is a texture you want to disguise you can sand deeper.

Use a paint with a contrasting chemistry as undercoat, EG: use an oil-based paint with an organic solvent, or a shellac based sealer with an alcohol solvent if your main coat has a water solvent.

Answer 3

The reason for this is likely you’re not using a stain blocking sealer / primer.

Answer 4

A simple solution is to prime with a silver or light grey paint. If it’s a large area you can just prime the whole thing with it, or spot prime just to cover the bad areas. For just a few small spots you don’t have to get a whole can of grey paint to do it, just use a little bottle of craft paint. It works just as well. I’ve done this for over 25 years on plywood, trim boards, furniture, ceilings, doors, you name it. I have never had anything bleed through.