I need to fuel proof my cub's firewall. It looks like the ARF factory applied something, but I'd rather make sure its done well. I've heard that thinned out epoxy works well for this. I don't need any color - in fact, it'll all be under a cowling anyway.

What is the best way to fuel proof engine area? If the epoxy route is best, how to I thin epoxy? How far do I thin it? What else am I not thinking of?

I use denatured alcohol and thin it close to a varnish consistency and brush it on. 30 min or 5 min works fine.

I use epoxy as well, but I prefer to use heat to thin it with. Use 30-45 minute epoxy and place some on the area you are fuelproofing, and hit it with your heat gun. It will thin down to the consitency of water and you can easily brush it in place. I prefer to use the heat because it doesn't change the chemical breakdown of the epoxy. Using a thinner it is possible that you can change the epoxy to the point that it will never set and you'll wind up with a gooey mess that will need to be scraped off and you'll have to start over. Yes, I know that many use thinners with no problems and I have done so in the past. But like I said, it's possible to mess up the epoxy. Plus, using heat you can get the epoxy to a thinner state that will allow the epoxy to be easier to spread and will soak deeper into the wood which will improve the fuelproofing of the area you are working on.

Hope this helps

Ken

+1

Ken, you are exactly correct using heat to thin epoxy.

I've never had a problem using alcohol so the heat solution never came to me. The gooey mess can result from using household alcohol instead of denatured due to the water content.

Good luck with your project either way you choose.

Jeff

So just mix the epoxy as usual, paint some onto the firewall, then hit it with a heat gun and spread it on? How many coats do I need to apply?

Yep, that's all you need to do. Mix up the epoxy and hit it with the heat gun. You'll see if thin out and start to run like water. Use and epoxy brush to start painting it the entire surface. Now bear in mind that even thoughyou are going to be working with 30-45 minute epoxy you will have nowhere near that amount of time to work with it before it kicks off and sets. There are two things that affect the cure time of epoxy, mass and heat. A mass of epoxy in a cup will set off quicker than if it's spread out. And heat will shorten set time as well. So when you start to heat it keep in mind that you may only have 10 minutes or less. If you have a bit area to cover you might want to break it up into several different applications.

Normally only one coat will be enough. As long as you brush it in well it should cover everything. As I said above, thinning it this way will cause it to soak deeper into the wood so you can usually use less epoxy to cover a given area. This is good because epoxy is heavy, and the more you use the more weight you add to the plane. One should be enough to cover, but if not you can always do a second coat.

Pull out a scrap piece of ply and take a practice run with this method. You'll see just how easy it is to do.

Ken

While you are at it, make sure that there is a drain under the tank. If the bung lets loose, or the tank splits, and you don't have a drain for it to run out, you will really soak the fuse. My last "event" on the tank was the bung going loose in the tank. Idiscovered it after three days of engine not running quite right and then oil on the laundry room floor and rug when I brough the plane in side and tipped it to clear the doorand itpoured out the puddle of fuel that had accumliated. The wife just loved that.

When I build, I put a servo gromlet through the bottom of the fuse at a low point so any leakage will show up there. Didn't on this ARF though and now it's await and see how long it holds together now that the fuse is fuelsoaked.

Don

I heat my epoxy in the microwave oven for about 20 seconds. It runs like water and cures normally.
Ijust read RCKens posting about heat speeding up the set time and yes, Ihave seen that as well.

I'm going to chime in with using Acetone to thin the epoxy. Acetone is specifically identified as being a thinner for epoxy, it doesn't change the cure time like heat does, and it acts as a carrier to pull the epoxy into the wood fibers. It evaporates away, so it doesn't leave the epoxy gummy or porous like alcohol can. Just use it in a well ventilated space. Mix it about 25% acetone 75% epoxy, and you can apply it with a brush, a small rag, or even a gloved hand. Pour it into hard to reach places and roll the fuselage around and it will seal joints and seams.

Brad

If you go the heat route with epoxy I highly recommend doing so in a well ventilated area.

Bryris

Thinned out expoxy does work well and I have used it on several of my models models.  Use 70% to 90% isoproply (rubbing alchol) from the drug store for thinning.  I'm not sure of the mix ratio (depends on the epoxy) so just add a little at a time until you get something with the consistancy of paint.  Thinning adds to the dry time so plan to leave it over night.   

Another option is to by a clear dope (airplane paint) from the hobby shop.  Typically available in clear or colors.  Read the lable to make sure it is fuel proof.   If you by the colored dope it can be used to cover small damaged areas that crop up after some flying. 

One thing I've started doing before I fuel proof.. I draw cross hatches to mark center line on the firewall. I also write any thrust info on the firewall as well...

THEN I apply heat-thinned epoxy.

Once done, the writing is sealed in. and it'll always be there, if you ever need to re-engine, or sell the plane... etc...

Just a thought.

Balsarite works well, 2 or 3 quick coats, straight from the can. I use an acid brush (the same brush you usually use for epoxy). Dries very quickly.

Ken gave you some good advice. I've seen to many gooey messes when people tried to thin the epoxy, especially with medicinal alcohol. There is also a "finishing epoxy" available which is quite a bit thinner (flows and brushes easily) that you can use. Heat and slow epoxy (30 minute or slower) works best and always gives good results. It is best to heat it after application, not in the mix pot. It also helps to have the surfaces being painted warm prior to application.

If you do thin epoxy with some kind of solvent it is critical that you mix the epoxy first then add the solvent. Also epoxy does not like any water mixed in so make sure your solvent is kept sealed.