Hot coating ply wood ?

[pescaloco]

hey guys,

For priming or hot coating wood, which will consist of clear Doug Fir and ply wood I was told long ago to dilute the polyester resin with acetone, catalyze hot and float a coat or two onto plywood prior to any laminating to seal the wood.

I know all the stuff with epoxy, but will probably opt for an iso polyester or vinylester.

Excluding epoxy on the end (edge) grain of the plywood which (I will do) on the the cockpit sole & fore deck edges. How do you recommend to pre-seal the wood.

Thanks in advance

[ondarvr]

While thinning resin (any type) for better penetration seems like a good idea, it really doesn't do much and may create other problems.

I frequently see people discuss adding 40 to 50% of some type of solvent to resin and then letting it soak into the wood, while it will penetrate slightly deeper, it also drastically reduces the physical properties of the resin.

The resin will be weaker, less water, UV and crack resistant. There can also be a great deal of shrinkage, meaning if the resin did fill a void or space in the wood, it will shrink and most likely still leave a void.

The best method would be to use a resin that had a low viscosity to begin with and use that for pre-coating the wood.

A small amount of Styrene (less than 5%) could be used and not create noticeable problems, but too often 5% turns into 50%.

Just pre-coat the wood, more than once if needed and allow the resin to at least gel before laminating over it.

[ondarvr]

I almost forgot.

Don't hot coat, you're better off staying with in the recommended range of catalyzation, if you don't it will again mess with the chemistry and reduce the physical properties.

Catalyze at the low end of the scale, this should be around 1% and mix it very well.

[mark775]

I was told this hot-coat thing 40 years ago and even believed it until my projects started coming apart. I don't believe acetone has a place here, at all. Resin manufacturers have a dedicated thinner. Why the resistance to epoxy?

[PAR]

Penetration has very little to do with the ability of a coating to seal wood. The moment you dilute a resin, you decrease it's ability to seal wood. In short, trying to increase penetration with solvents is self defeating.

Don't mix epoxy coated end grain with poly coated flats. Poly doesn't stick worth a damn to epoxy. Pick one, knowing full well if it's poly, you will be letting moisture get at the wood, unless it's an 1/8" thick laminate (or more).

In the nearly half a century since you heard about the hot coating to seal wood technique, we've learned considerably more about how things work. Hot coating is useless, as has been born out by thousands of repairs. Ditto screwing around with catalyst ratios. There's one ratio that will offer the most protection you can get from the resin selected. Use it, or anything else will change the chemical composition of the cured matrix and it will not have the physical properties you're looking for, including water vapor penetration.

[mark775]

That's how you say it when you know what you're talkin' about!

[lymanwhite]

'Hot coating is useless, as has been born out by thousands of repairs.'
'Hot coating' that I am familiar with is the process of wet coating a part so that a laminate or a thickened epoxy mixutre will mechanically bond and penetrate cracks or an uneven surface better than if only using the thickened mixture. I do agree that excessive thinning would weaken the bond, especially with 40% acetone. I've seen epoxy thinned 5-10% with a med. or high grade paint thinner (PPG 885). This would seem to promote better penetration, if followed with the regular ratio catalyzed resin.
After cleaning out a core between two skins, as in mounting a deck fitting, I have often wet coated the area before putting the 'hard spot' material in... We've referred to it as a 'hot coat' because it starts to kick as the thickened material is added.

[PAR]

Look, the chemistry is fairly easy to understand, if you have ahead for this sort of thing. ANY dilution of a resin system (epoxy, poly, whatever) will weaken the molecular bonds, limit cross linking and generally weaken the cured matrix. This isn't opinion, but well proven in testing. This in light of the fact, that water proofing isn't dependent on penetration, but the quality of the coating and ability of the coating to remain intact and resist moisture vapor ingress.

Hot coating by my experience, has been alterations to the resin/hardener ratio to "home brew" some sort of "special" quality into the cured goo.

Something seemingly as benign as a 5% thinning of standard laminating epoxy resin, can result in upwards of a 40% reduction in strength and a substantial reduction of water proofness, so what have you gained by thinning the goo. Polyester and other resins have similar difficulties when shade tree chemists start fooling around with things they don't fully understand.

[pescaloco]

Guy's thanks for the feed back, for me it was over 20 years ago I was told to thin down the resin, and yes it was and is a very shade tree afair.

Being older a hopefully a little wiser these days I figured it was time to get the scoop before diving into a big project like fitting out a complete 27ft skiff and doing it wrong.

It was asked "why the resistance to epoxy."

1. the hull / stringers / bulkheads transom wall, are all polyester those being the most important stuctures in the boat it seemed pointless to do the rest in epoxy.

2.In the past I have had bad luck with epoxy, it seems harder to work with for laminating. Ofcourse that was years ago and I know there are lower viscosity epoxys these days that wet out well.

Thanks

[lymanwhite]

'Look, the chemistry is fairly easy to understand'...
Basically right. However it is no good to confuse folks with syntax (hot coat / wet coat), as there is already enough symantic confusion. I was simply trying to clarify an issue. I continue to find good uses for unthinned hot coating / wet coating with epoxy, having never worked with polyester. And, if the surface is very difficult to get to a 'slightly' thinned mixture may also be warranted.

[ondarvr]

Hot coating typically refers to adding more catalyst (sometimes far more) than recommended to speed the cure of the resin, this is the bad practice most are targeting. Pre-coating, or wet coating would typically involve catalyzing at the correct ratio and allowing it to soak in (a very good practice).

I would agree that adding even 5% styrene can create problems, in most polyesters we can see a difference at 3%, but typically 5% can be added before physical properties begin to degrade to the point were there are issues. This all depends on the exact resin formulation though. I always "recommend" adding 0%.

[mark775]

Styrene is great for cleaning scissors and rollers!