VARNISH

[riske]

Hi Folks,
I am an amateur boat restorer and a professional automotive re-finisher. I am about to experiment applying automotive urethane clear on some mahogany Chris Craft parts.
I will apply the urethane with a HVLP gravity feed gun over some fully cured, properly prepared and sanded varnish. I will also attempt to apply the urethane over some relatively fresh (1 week) varnish. I will also apply the urethane to some mahogany which has no varnish but has been stained with filler stain.
Has any one else out there tried this? If this works, it could revolutionize the whole finishing process.
BILL

[Bob Cleek]

If it works, it woulda been done before... right? Don't go there! If you have varnish down, then varnish it is. I doubt if any varnish coat will survive a coating with PU, given the solvents in the PU. PU sucks as a clear gloss coating on wood anyhow. It lacks the flexibility and breathability you need for a wood finish. They spent a few hundred years figuring out what works when painted on wood. It isn't like somebody's suddenly invented a new mousetrap already. Sorry to pop your bubble, but think of it this way... I'm saving you a big stripping job! LOL

[JimD]

polyurethane clear topcoats are now frequently used as a uv blocker over epoxy, but it sounds all wrong for covering varnish as Mr Cleek says.

[Rocky]

Didn't someone do a whole Chris-Craft with automotive clear a while back? It's OK over CPES, stain, or bare wood, but not over varnish, is that what you're saying?

[On Vacation]

This is not an autobody. NO, NO, NO, and if you do it, and keep the boat for any amount of time, you will have a sure total loss of work.

[Scott Rosen]

I disagree with you guys. Two-part LPU's are applied over oil-based varnish with success. For example, Smith & Company will provide instructions on how to apply their FiveYearClear over varnish. Sterling will give instructions on how to apply their clear LPU over varnish.

The LPU film is much more durable than varnish and applied over varnish will reduce the amount of maintenance. The varnish will still eventually break down, but the surface will not need recoating every year.

Riva of Italy uses clear LPU on their mahogany runabouts. I don't think they use it over varnish, though. Their system has a high-build undercoat that has very little UV protection. After about 6 to 8 coats of that (multiple coats can be applied in one day), they apply a few coats of the topcoat, which has high gloss and UV protection.

The biggest drawback with overcoating varnish with LPU is that repairs to the finish are very difficult to make. And no matter what, you will eventually need to make a repair.

[Rocky]

This has been discussed before. Someone else did a new swim platform with it and was very happy with the result. Let's not be so hasty without really knowing, shall we? They didn't have clearcoat in the good old days. As a public service, I shall shortly spray the cheapest clearcoat I can find on the outside of my transom and generously share the results of my research with the public.

[Peter Malcolm Jardine]

Here is my experiment.. (with some initial advice from Conrad...)

I disassembled my swim platform and reglued and screwed every joint. I steam bent a strip around the ends to cover end grain and epoxied that in place as well. I applied a 1 1/4" oak half round screwed, epoxied and then plugged with teak plugs for contrast. I sanded carefully. I applied two coats of CPES to seal the platform (teak) and then sanded lightly. Two coats of two part clear was applied with 25% more flex agent than normal. (normal being 10% I'm told) I brought it back from the bodyshop, and block wet sanded the entire platform. I then checked it for any areas or cracks that might have been missed and using a 14 gauge needle and syringe injected a high density epoxy mix into the remaining cracks. The platform was then sent back for two more coats of clear. I have picked it up, and it looks great. I plan to carefully bed the platform brackets in goo, and I forgot to mention that the holes for the brackets were predrilled so they could be sealed. Life expectancy? I guess I will find out, but Conrad has reported good luck as long as the preparation is done properly. smile.gif

[NormMessinger]

Where is Wild Wassa when we need him?

[thechemist]

A Practice is something that works, or appears to work, sometimes for some people.

Witchcraft, for example, is a Practice.

Science is reproducible, independently verifiable knowledge.

Technology is the application of Science to achieve some goal or solve some problem.

Varnish presumably means a clear finish created by oxidation of olefins to give ether crosslinks, said olefins being those in a class of unsaturated natural oils called "drying oils" because they were long ago observed to "dry", meaning form a dry film.

Tung oil is one such.

That kind of Varnish has no hydroxyl groups to which isocyanates may form covalent bonds. Those are the strongest kind of chemical bond........it's what connects atoms into big polymer molecules.

It has polar groups to which other polar materials may form polar bonds, a sort of half-way bond.

It's better than nothing.

Such adhesion-technology is fair in some circumstances, but may be best termed a Practice when used by the average consumer on consumer-type products.

If you want results at the level of Science I would recommend that adhesive bonds in coating systems be only [covalent] chemical bonds wherever possible. The manufacturer of a product can best advise you on this point, if you can find a chemist there.

[Gary Bergman]

Been there, done that. The overcoats flash off kinda funny, you can't fix dings, and we all strip varnish regularly. Why hide from it? Learn to love varnishing, it's a zen thing, kinda....