Ward,
I used to use Lustrekote over dope for all of my models, lot's of them. From my experience you can safely spray Lustrekote over any brand of dope and any vinyl or waterslide decals, including all of the Sig ones. I did occasionally have the gloss clear come out with actual bubbles in it about the size of a pinhead that would land on the surface and then stay there because it dries so quickly. I had it happen often enough that I quit using the Lustrekote clear altogether. I then went to Midwest's "Formula U" clear (over Lustrekote over dope) until they discontinued the Formula U. I then started using Minwax aerosol urethane (over Lustrekote over dope) and it worked nicely but was quite brittle after a while, like less than a season. Any aerosol urethanes should be OK over any decals from what I've seen.
You
CAN NOT however use dope in reverse order over any of these. It will absolutely not stick at all and the more you mess with it the uglier it gets. Been there, done that...once. (It didn't work the second time either
) So if you use Lustrekote the only repairs you'll be making will be with iron on film or completely stripping everything back down to the wood. If you get out the iron you have to really watch the heat or it will goo-up, but it does work.
The best move
I've ever made though is finally "biting the bullet" a little over a year ago and investing in a compressor and a couple automotive detail guns and now spraying Brodak dope. I could probably have six complete spray setups for the money I spent on Lustrekote over the years.