Coating wood for paint prep?

[Jeff Wohlt]

What is a good coating that I can get at the LHS...or anything else that works, that I can spray or brush on balsa or ply to seal and get ready for paint. Could a person spray clear coat several times and then sand? I used thinned epoxy on one and it sure would take a lot of sanding to get ready for paint...in this case I left as wood.

Any help would be appreciated...I want easy and nice finish for painting.

[ccboatworks]

The problem I've had w/ clear coat is that it doesn't stick well to the bare wood. Have you ever tried brushing on polyester resin or autobody spot filler(comes in a tube it's usually green or orange) both of those do a good job and sand much easier than epoxy. I use epoxy though cause I just love sanding :)


Chris
CCBoatworks

[eddieh]

Jeff, I Have actually had great sucess with a lacquer based sanding sealer used for floors, It completely fills the grain, it's clear, wet sands like a dream, and there is no problem with automotive type paints over it,,,, it is also fairly cheap, very light, and since it is made for floors, very very durable (Hard) not quite the same as epoxy, but if your not glass clothing, why freight the weight,,,, as I am learning here,,,, keep it simple, keep it light,,,,, cheers ed

[Doug Forrester]

This is a huge subject with many considerations. We use balsa on our hulls because it's light, or can be light. Actually some balsa can be extremely hard, dense anf HEAVY. I seem to remember that balsa wood has the greatest variation in density of any wood. The lightest grade (4-6 lb./cu. ft.) of balsa is very soft wood. The problem with balsa as an exterior surface is not just a matter of filling the grain to get a good poaint finish but also using strong enough balsa to have adequate structural strength and most important, having a hard enough skin to resist dents and abrasions.
Using a liquid finish such as resin by itself, laquer-based sanding sealer, dope, polyurethane, etc. is that by the time you use enough to fill the grain of the wood and give a hard, smooth surface, you have added a fair bit of weight. Also the hard surface is quite thin and because the balsa underneath is still soft and crushes easily, unsightly dings (in your fantastic paint job) still happen all too easily.
I have found that the secret to using balsa on raceboats is to skin any exterior balsa surface with any of several different materials.
1. Glass cloth (.5 oz - 2 oz) and epoxy resin, thin CA or even
nitrate dope
2. Thin plywood (1/64" or 1/32")
3. Silkspan tissue adhered with dope, white glue or thin CA
4 Nylon stocking (attached as above)

All of these surface laminations serve to provide surface punture resistance and as a bonus, automatically fill the grain. Also by laminating a fabric or other fibrous material on the outer surface it makes it feasible to use a lighter grade of balsa than might be prudent otherwise. On smaller boats, I tend to lean towards the use of Silkspan tissue and clear dope for its light weight. On larger boats where more strength is required, I use glass cloth (.75 oz - 1 1/2 oz) but not using epoxy. It is very difficult to use just enough resin to adhere the cloth. If you use enough to fill the weave of the cloth, it's too heavy (even if you squeegee it). I use thin CA to adhere glass to the balsa. It doesn't take as much as you would think. If you incline the wood/cloth surface and lay a bead of CA horizontally across the top, the CA will run down and wet out the fabric. You will be amazed at how little it takes per square foot to wet out the fabric. In doing this with CA. there is a lot of CA fumes produced. Make sure you do this with adequate ventilation. Some people (like Andy) are extremely sensitive to CA fumes and can't use it at all. 3M does make organic vapour face masks to prevent this (I have used them and they do work). Also in applying the glass cloth to the balsa, first I dust the balsa surface with 3M Spray 77, and then I stretch the glass cloth over the balsa and lightly rub it down. Then I CA it.
After the CA has cured, I trim and sand the edges smooth. Then you can use a lightweight filler (latex based) to fill the remaining weave of the cloth and sand it smooth ready for priming.

[Jeff Wohlt]

Thanks for the responses. I really like that I can use some CA on this. I would also like to put it over my stuffing tubes in the hull for extra strength. I also could try some medium for this I guess.

Thanks...Can't wait to do build some wood boats which I think will be the ones I actually keep for years to come as a real tribute to boat modeling.

[Paul]

Hello Doug,

I've been struggling with this problem for a while now as well. I've used every technique mentioned above with the exception of nitrate dope and I'm starting to lean toward lamination of lighter balsa with ply skins. The sanded ply seems to take the Krylon primer nicely and I haven't had paint peeling problems with this method.

I do have a question for you, the one time I used the CA/FG cloth method I was very disappointed with the end product. The amount of work required to fill the weave and smooth it out made the project less than enjoyable. You mentioned using a latex based filler, in specific what type of filler is it and how is it applied?

Thanks, Paul.

[Doug Forrester]

Paul
I've used a number of different light fillers,some mean't for regular wood use and bought from Home Depot (I don't remember the brands) but the one that I currently use and works quite well, is made by Hobbico (I think Great Planes/Tower, etc. conglomerate) and is called Hobbylite Filler. It comes either white or balsa coloured in a plastic sm. tub/jar. It is thinnable with water although they caution against too much water (causes shrinkage when drying), I thin it a little bit and apply it with a spatula (it's a little too thick to use a brush) - an artist's palette knife works very well. The stuff really is as light as balsa but also quite soft so you have to be careful sanding and just use relatively fine paper. It cuts really easily (but that's a bonus for those of you that object to lots of sanding).
For fillet use (thick), it should be built up in layers (each drying before the next one is applied) or you risk it shrinking and cracking upon drying. Thick deposits can be hardened with thin CA after they are dry.

[AndyKunz]

The filler he mentions there is available in white and balsa colors from Hobbico and Carl Goldberg.

You can get it in a variety of colors from your local art or craft shops, where it is sold under the Crayola name.

Andy

[Paul]

Hello Guys,

The filler he mentions there is available in white and balsa colors from Hobbico and Carl Goldberg.

If that's the Model Mate Superlite filler I've tried them and had horrible luck with the stuff. I gave up on it because I just could not get a good finish using it. Maybe I'm using it the wrong way, if you guys are thinning what consistency do you make it?

Thanks guys, Paul.

[AndyKunz]

I only thin mine when it gets lumpy in the can. Then I take it to about stock thickness.

Did you wet the wood prior to applying the filler? It sticks better if you do.

You need to have more on than just enough to fill. It _has_ to be sanded down to the wood to get a good finish. Also, you need to use filler over it (dope, etc) to harden the surface and fill the pits.

Andy

[Drobie]

Originally posted by Doug Forrester
If you incline the wood/cloth surface and lay a bead of CA horizontally across the top, the CA will run down and wet out the fabric. You will be amazed at how little it takes per square foot to wet out the fabric.

Doug, do you use the thinnest CA for this?

[Doug Forrester]

Filler: What Andy says. Dope or thin ca over the filler as a last step after sanding very smooth.

Doug: Yes, I use the thinest CA to adhere the glass cloth. It doesn't fill any of the weave but it hardens the cloth, adheres it and hardens the surface balsa all at once. You then have to fill the weave until it's smooth. As I mentioned, I apply the filler with a spatula and try not to rework it too much. I don't worry about applying too thin a coat. I think trying to scrape it too thin is trouble. You end up sanding 95% of it off afterwards. It's not as much work as it sounds because it sands VERY easily.

[Paul]

Hello Guys,

Well I'll give the stuff another shot and see what happens. (got 2 tubs of it) I had not tried wetting the wood, do you wet down the FG cloth as well or just leave it dry?

I haven't worked with the nitrate dope yet either, what's it like? Hows the smell? Does it dry quick?

Thanks guys, Paul.

[Doug Forrester]

Paul
I did not wet the FG cloth - didn't seem to make any sense to do so.
Nitrate clear dope sure does smell like dope! Adequate ventilation is advised. Yes it does dry pretty fast.

I have also used conventional fillers such as Polyfilla, etc. They seem like they would be heavy but with lightweight cloth (under 2 oz.), the weave is so fine that by the time you sand the filler smooth there is very little of it left on the surface.

[Drobie]

Originally posted by Doug Forrester
Paul
I've used a number of different light fillers,some mean't for regular wood use and bought from Home Depot

My wife and I just built a house and the entire interior is finished in mahogany. I'm pretty particular about my wood.

I use Famowood for filling. It contains real wood sawdust mixed with the coagulant so it sands very well and takes stain and paint great. Maybe available at Home Depot, but a place like Woodcrafters catering to the serious woodworker might be more likely.

And you know, when you use SAWdust on your boat, it will result in magical speeds.

[AndyKunz]

Sounds like a nice house! My dad's uncle built a house entirely of cedar cut from the property. That was 50+ years ago and it still smells sweet! (Oak floors and beams were harvested on it too).

Andy