Yeah, what Mr. Jim said...
I seem to remember trying that on my Stuntman 23 (back in 1983...so give my brain a break, please). I doped the whole plane to seal the wood, and papered the wing, stab and rudder for strengthening. I also want to be clear that I tend to build too heavy, and have learned what little I know from first hand, old school trial and error. I had no peers to experiment with, nor a mentor to show/teach/guide me. So this means that what I "know" may not be optimal, and quite possibly just mis-informed belief. Oh, and I build/fly 1/2A...in short, when I begin my Brodak Cub, or my Veco Tom Tom, I may be re-thinking the same questions...
What I learned was that thinner-only would quickly saturate the tissue, and begin to flash off in different places before really affecting underlying dope and not sticking the tissue down very well. The tissue felt like an old, dry scab ready to fall off the injury. My recollection is that whatever stuck wasn't enough to keep the tissue down firm, and the tissue skinned-over enough that any more dope couldn't get through the tissue and below. In fact , the tissue would almost "de-laminate" if worked harder.
Using thinned dope allows for saturating the tissue and keeping it wet long enough to soften underlying dope for a really good, firm stick, with dope throughout the tissue grain and below. Subsequent dope applied much like in the late stages of sealing balsa would behave much like we expect it to go.