Started gluing my own tubulars this year and riding them every day. Could kick myself for 20 years of clinchers - used to have the shop do the gluing and only ride them on race day. I have 404s with Vittoria Corsas - followed both mfrs instructions for gluing (three layers on rim, one on tire) - and got a very nice installation. Time to replace rear tire is approaching. I know I don't have to clean the rim down to new this time around, but how many tire changes/layers of glue do folks recommend before doing this ? Thanks.

Keep adding layers. If you remove a tire and the glue on the rim is chunky and irregular in height, then it's time to clean it off.
In general, more layers of glue is lower in rolling resistance (up to a point, I'm sure).

Doubt there is a set number of changes for replacing the glue layer. Probably total thickness is a better measure. If the dry glue is 1/4" thick, then its time to start over and clean the rim. If the glue is 1/8" thick, then add another layer. As already mentioned, if the glue is lumpy, comes off irregular and leaves spots of thick glue and spots of no glue on the rim, then probably start over. Having a consistent thickness of glue is important for rolling smooth. When I glued tubulars I put two coats on the rim and let them dry. One coat on the tire and let it dry. Then when actually mounting the tire I put a fresh coat on the rim and stretched the tire onto the rim. Held just fine. Used Continental tubular glue.

I would tend to agree with thisisatest.

This. Knock off the big chunks and keep gluing.

Glue sticks better to glue than to AL (or carbon)

M