Tubs

Aluminium rims so acetone fine

No need to bother trying to clean the rims of old cement. A bit of a rub-down with a firm metal brush and a good wipe with alcohol to remove the dust and all the loose bits is sufficient. If you want to remove the old cement your better off with mineral spirits and ethanol than acetone.

Seems I'm an "idiot" possibly i will end up covered in cement is "idiot" better or worse than idiot? I must know ;)

Don't need to use a lot of cement. A priming with mastic on the basetape and a thin layer of fresh cement on the rim bed is generally sufficient. Don't try to rush things. When using rim cements such as Conti and Mastic One you can wait until the glue has firmed--- still tracky to touch but no longer flowing. Even without fresh rim cement its amazing how well old mastic holds a spare tire..

Anyhow as long as the tire doesn't roll off it's for street use in good weather (79 ciocc with campy SR group going on) I'm happy.

When a tire rolls-off its clearly "too late". Proper mounting is, however, not just about keeping the tire on the rim but also about reducing tire movements on the rim. Tire creep produces not just rolling looses but can result in tire defects.