In an effort to get the hull of my Brede lifeboat as smooth as errr, a lifeboats' bum, I'm using a combination of model filler and spray filler. The former is cellulose based while the later is acrylic I suspect. They don't seem to like each other much.
Cellulose fillers such as knifing stopper are notorious for shrinking as they dry and I suspect that is what's happening here. The filler drys and cracks the spray filler on top. Stupid stuff. How the car repair industry gets on with it I don't know.
Anyway, as it's been too cold to spray I've been power sanding the hull and then recoating it with filler. Again and again and again. Each time duff sections are taken down to the fibreglass leaving only vestigial traces of filler and every time I do this things look a little better. Each time I think I'm there and then decide there is just a little more work to do. The hull is now in grey non-filling primer with Isopon P38 filling tiny marks (not as fine as cellulose filler but non-shrinking) and once I've sprayed I'll have another sand.