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I'd like to treat some wood (raw pine) with a water-based class A flame retardant (Flameseal WSA), but also stain it with an oil based stain (garden variety Minwax or Varathane).

I'm a bit unsure of compatibility, and the need for a barrier layer (e.g. shellac) if any.

The FR datasheet states:

This product may be used on most types of wood as long as penetration occurs. The product may be stained with a compatible stain or top coat after treatment. However, many stains and top coats are flammable and can adversely affect the flammability of the treated substrate.

The priority is FR effectiveness over looks, but I'm concerned because the flame treatment is penetrating, then if I have to put a coat of shellac over top, I'm thinking any stain on top of that isn't going to penetrate well. I'm also concerned that, even worse, the stain will never really dry properly. The wood will be used on a ceiling so at least any finish doesn't have to stand up to heavy wear and tear, although if it stays sticky forever it will attract dirt.

So my main question is: How can I apply both the water based FR and the oil based stain and get the most out of both?

My second question is, given the last sentence of that quote above, should I be considering a water-based stain or some special FR stain instead of my go-to oil stains? Does anybody have any experience with this?

I only actually need a class B rating (these are ASTM E84 ratings) so I do have a fair bit of leeway with reduced FR effectiveness, if it matters.

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    This sure reads to me like it must go on to bare wood. Since most stains won't won't work properly over something else (unless it's very thin and sort of disappears into the wood?) I'm thinking this is calling out to use "gel stain" instead as the colouring agent. But this, and any varnish applied on top, are flammable...... but then so is almost every finish!
    – Graphus
    Commented May 28, 2019 at 6:55
  • @Graphus From what I can tell it's thin, clear, odorless. I've never used Flameseal's stuff before. I guess I'll know more when the shipment comes in and I can test it. I don't have the equipment for a flame spread test but I guess I can do an ignition time test at least, and I have a friend in the FD that can probably give a credible thumbs up or down. No idea what to do if the stain looks crappy or doesn't dry though. I'm going to try some minwax water based stain as a test. Gel stain is a good idea too if it doesn't cancel out the fr; thanks.
    – Jason C
    Commented May 28, 2019 at 19:34
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    FYI, any informal tests you do should not be used as benchmarks. The certification these materials go through to get their ratings is pretty specific and done under controlled environments. As long as you follow the directions provided by the vendor you should be ok. The idea here is not to put on any finish that behaves like an accelerant. It'll be cooked off in seconds, and then the retardant just has to keep the wood from off-gassing sufficiently to actually set itself on fire for X minutes under Y heat.
    – user5572
    Commented May 29, 2019 at 14:38
  • Yeah. I am waiting on a reply from Flameseal on oil vs water stains. I was all set to just go with water based stain; but then I saw another water based FR that called for oil based stains under the rationale that the water in a water based stain is a solvent that can destroy the FR. Then, when considering a varnish, I found pdfs.semanticscholar.org/4a24/…, and apparently polycrylic generally performs more poorly in flame tests vs polyurethane. So, it's confusing. Waiting for their reply.
    – Jason C
    Commented May 29, 2019 at 16:39

2 Answers 2

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I spoke directly with the Flameseal manufacturer rep and was told that they won't warranty their product if any stain is applied over their treatment. In my case, I would not be able to get this by a building inspector for the application intended, so have looked to other alternatives, and coming up empty.

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  • Flameseal make fire retardant paint, and they claim they can provide many colours. "fireproof stain" as a general web search came up with a few hits. You may be a bit limited to the exact finish, but it looks like there are at least some basic finishes that give you what you need.
    – user5572
    Commented Oct 25, 2019 at 17:19
  • Thanks for coming back to follow up! Even better would be if you stop by again with the results of your further search.
    – FreeMan
    Commented Oct 25, 2019 at 18:34
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From the link provided:

The product must penetrate the wood and be allowed to thoroughly dry to be effective.

and

The substrate must be able to absorb the water based chemical to provide fire protection. A simple guideline test is to place a few drops of water on the material – if the water is readily absorbed

It almost certainly needs to be applied to bare wood to be effective. It is not clear how the effectiveness would be impacted by applying over some finish, or how that might align or not with "ASTM E84". But my guess is that, legally, not following the instructions to the letter means you don't get the liability coverage you want by using this stuff.

Your research, then, is what would be a suitable finish that would go over the treatment. This is the main question you are asking, I suppose.

I note that many manufacturers sell "non-flammable" finishes if you search for those terms online. I take it to mean that, once cured, they do not contain any appreciable accelerants. This is probably most of the non-oil-based, polymerized water base finishes out there.

But, like I said, there are companies that specifically advertise non-flammable finishes. I bet you could short-list some of those and even ask them how they fare as a top-coat over the fire retardant you want to apply.

It occurs to me you can also look at the MSDS for any finish you are interested in and look at section 5, "Fire AND Explosion Hazard". Most or all of those should be "N/A" or "None" for any non-flammable finish.

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