I'm in the process of building a Sig LT-40 ARF. I'm going to reinforce the stab and fin with 3/8" tri stock. My question is should I use CA or epoxy to attach the stock. TIA

if the tri-stock is balsa, then I'd use CA to save weight. If it's hardwood (like basswood) then I'd use epoxy. Epoxy at the tail will add some tail weight--be careful about how much you use. Why are you beefing it up? I've not heard of this being a weak point on the plane.

It is a weak point. The fin is only attached to the top sheeting. Mine cracked after many bad landings. use epoxy or CA ether will do the job.

LowLevyl's kadet's fin was weak too, he beefed it up with triangle stock. I'm sure he's done his new one the same way. It seems to be a known flaw in this plane...

Forgive me if this is a dumb question. Do I need to remove the covering from the fuselage before I apply the tri stock with CA? Also, should I use thin, medium, or thick CA?

Yes...peel back (carefully) the covering where the triangle stock will go...so the wood is contacting wood....

I used epoxy on my LT-40....
I save CA for my foamies....

I then used some spare covering to cover the triangle stock so it matches the plane

You will need to remove the covering in the area the tri stock will go. I suggest thin OR medium CA (thin is my choice, but others may have different opinions).

Once you have the tristock CA in place, dont for get to protect the new wood. I would put some AeroGloss paint on in (AeroGloss is fuel proof) to protect the wood and help "blend it in" to the rest of the plane.

I would prefer medium CA as it will allow you just a bit more working time. With thin, place the tristock up there, and then wick the thin CA into the joint. Any irregularities in the surface, however, will not get glue. I'd go with the medium. Then, recover it with monocote of the same color, or paint it, but monocote would be easier IMO.