Sorry, can't speak to the efficacy of the water-based cement. I'm now using the standard oil-based Dap/Weldwood product, again based upon the recommendation of my interiors guy.
I don't think painting
the inner shell is necessary, if you have a good, clean and smooth surface with which to work. Frankly, I paint
mine first to help me locate any possible problem areas, and to seal any really raw areas of fiberglass where aggressive sanding, et al, may have been required to get the old crap out of the way.