I make cowls using styrofoam plugs. Generally I put on about 3 layers of 4 or 6 oz open weave glass. Between layers 2 and 3, I fill in any voids with spackling and do a finish sanding job. Then do layer 3. I have never sealed the spackling with anything. Have'nt had any problems yet. I will be doing a complete glass job pretty soon on a wing mfg P-38 and will use spackling to do the finish sanding job like I always do. The important thing to remember about spackling is to sand 99% of it off. I slop it on liberally but sand most of it off. I dont do fillits with it. I used spackling once with ca as a fillit and found that it was brittle around a squared edge. Was used to fair a wing LE into the fuse on a P-51. The cracking was not a disaster, just a little bit. Just made me mad after all that work to make it perfect. Still havent done a perfect plane <g>. Just my experience with it.
Edwin