Painting a window sill - a question

XrayDave

XrayDave

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Strange title, I know.

Here's the thing, my daughter has just painted a stained wooden window sill.

She's given it two coats of undercoat but something is bleeding through and I'm wondering what we can do to stop it - she wants to gloss paint the windowsills.

I know for knots there is a special 'knotting' kind of paint that prevents this sort of problem - is there something similar for stained wood?

From what I've seen so far, it seems that sealing products can't be overpainted - hence this question.

I'll post a picture or two tomorrow.

Thanks in anticipation,

Dave
 
Is the window sill softwood? Patent knotting is really designed for use on softwoods which will subsequently be primed then painted, but I wouldn't expect knotty timber to be used for something like a window sill which would subsequently be finished with a clear finish (i.e. a lacquer)

If it is softwood, and the problem is down to knots, and the final finish will be painted, it should be possible to sand the affected area(s) back to the bare wood, treat them with knotting, prime and paint. In effect patent knotting is really just French polish (shellac dissolved in methylated spirits) - the red brown colour is because they use a (naturally) coloured shellac such as ruby shellac (in a production environment this is a useful, visible indicator that knots are treated) , but if you are concerned about bleed through there is no reason why you can't just use "white" French polish instead of knotting (which despite the name is actually clear)
 
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Thanks for the replies.

To clarify, the window sill is in a conservatory, not a kitchen or other room.

Not sure whether it is softwood - the answer is possibly *shrugs*

Can't see much evidence of knots however ...

XRD
 
Is it hardwood or softwood? I suspect that it is a hardwood and that the finish might therefore be an oil finish of some description, such as teak oil (is the wood a reddy brown colour?). Before painting you normally try to remove as possible of an oil finish using an appropriate solvent (finding the right one can take some trial and error - white spirits, methylated spirits, naptha, etc are all possibles) on a well washed white cotton rag (old T-shirt, etc), then seal it (again with a coat or two of French polish). Shellac in French polish is very good at stopping incompatible finishes affecting each other as well as sealing stuff like this - but you really do need to knock back the oil a bit before applying it. I suspect at this point your daughter will also need to scrape back all the paint then try again - but I would suggest trying a small area first to prove the technioque
 
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Yep, it is a reddish-brown colour.

Pics to follow later today.

Update: Pictures

These show the sills (or should it be cills?) after my daughter has ovepainted the wood with two coats of undercoat.

IMG_1117.jpg IMG_1118.jpg IMG_1119.jpg

Would either of these be any use to overpaint the existing undercaoted sills or is it still a 'strip back to the wood' job?

Stain Primer Sealer.jpg


Dave
 
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Zinser BIN is basically a derivative of good, old fashioned French polish (it is often referred to as a "shellac based sealer")...

But it's more expensive than French polish (maybe it's the titanium dioxide they add) and in any case I've been there (with French polish) already...
 
Thanks J&K.

However, it still doesn't answer the question I asked yesterday - namely "Would either of these be any use to overpaint the existing undercaoted sills or is it still a 'strip back to the wood' job?"

I don't understand the need to strip back to bare IF this stuff seals the wood suitable for overpainting. Surely if it is sealed, it is sealed??

(also you mentioned Zinser BIN - it doesn't say that on either product in yesterday's post)

Thanks once again,

Dave
 
I didn't mention Zinsser BIN, @Canyer did. My comment about BIN being a modified white polish were aimed at that.

XrayDave said:
However, it still doesn't answer the question I asked yesterday - namely "Would either of these be any use to overpaint the existing undercoated sills or is it still a 'strip back to the wood' job?"
Click to expand...
To which I'll quote a previous post of mine:
I suspect at this point your daughter will also need to scrape back all the paint then try again - but I would suggest trying a small area
Click to expand...

The fact that the original finish is bleeding through the applied paint indicates that the new paint needs to come off before you do anything else because the finishing regime just isn't working. To my mind there is no point in trying to seal the current mess in because that absolutely NEVER works (a bit like continuing dig a hole?). If the new paint is fairly recent it may just scrape off, or it may need additional help from a heat gun. Either way there will be less pain if it comes off

As to the original finish I stated that if it is an oil finish (as appears likely because you stated that the old finish was bleeding through), you need to remove as much of it as you can with a solvent first after scraping the new paint off, and after that you can seal it with white French polish (or for that matter Zinsser BIN):
Before painting you normally try to remove as possible of an oil finish using an appropriate solvent (finding the right one can take some trial and error - white spirits, methylated spirits, naptha, etc are all possibles) on a well washed white cotton rag (old T-shirt, etc), then seal it (again with a coat or two of French polish). Shellac in French polish is very good at stopping incompatible finishes affecting each other as well as sealing stuff like this - but you really do need to knock back the oil a bit before applying it.
Click to expand...

So I feel I have already answered your questions and to recap:

1. You need to get the new paint scraped off (and obviously to clean up any roughness with sandpaper)

2. You then need to degrease the existing finish and remove as much oil 'contamination' as possible.

3. (Additional, as no information was forthcoming before) If the finish is a glossy lacquer finish, it needs to be lightly sanded to provide a mechanical "key" to the surface (standard procedure for gloss painting to prevent peeling of applied coats - generally not required when going over oil finishes). In view of the applied paint this might not be a bad idea anyway. This is not a strip back to the wood sanding, just a light sanding with a 180 to 220 grit sandpaper to mechanically key the surface

4. Because of cross contamination between finishing materials a barrier layer of white French polish (or for that matter Zinsser BIN) needs to be applied and allowed to dry thoroughly before priming and glossing the woodwork

5. You can now prime the woodwork again...

Apologies if this wasn't stated clearly enough before
 
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@JobAndKnock,

First of all my apologies for incorrectly attributing the original comment about Zinsser B.I.N. to you. For some reason, I seem to have overlooked @Canyer ‘s post. Secondly my thanks for a very comprehensive reply and very kind of you to take the time to write that.

The fault in my understanding is mine, not yours. You were quite clear that the surface needed taking back and the original wood treatment removing with a suitable solvent as far as possible.

Having been pointed in the direction of French Polish and then Zinsser B.I.N. I thought I had found a shortcut. I should have remembered the old maxim “There’s no shortcut to a good job”.

I had thought that simply sealing this surface would suffice, but clearly not. Thankfully the sills (cills?) aren’t all that extensive so it shouldn’t be too arduous a task.

Simply thanking you seems such a poor way of repaying your kindness in taking the time and trouble to offer constructive help and advice regarding my daughter’s problem, yet giving you thanks is all I have to give.

So, a HUGE THANK YOU from me.

With best wishes,

Dave
 
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