Sig Lite Dope is what I've used in the past. But do thin it out. On a model of that size, you'll add unwanted weight easily. I would thin it out at least 50/50, and two coats might be all you need. When I was flying Indoor Free Flight, I found even Dope to be too heavy. I used to thin craft store acrylic to the consistency of milk and lightly airbrush the tissue. Dope is pretty good as an adhesive for tissue, but again, don't over do it. Here's a photo of a Peanut Scale Piper Tri-Pace I did. Total up weight was 5.3 grams without motor. A dollar bill weighs about 1 gram, so figure 5 singles would weigh approximately the same.