I do use internal fitted dust covers on most of the cars. However, the car needs to be absolutely clean before fitting the cover otherwise the abrasive dust will mark the paint on hard edges which means a machine polish to rectify. A fitted cover is literally "skin tight". Another summer related problem we have is swallows will fly into the workshops within minutes if any of the doors are left open looking for nest sites. Bird poo and cotton car covers don't mix ! If you buy an indoor cover it will probably arrive in fitted bag. When you take the cover out of the bag throw away the bag ... you will never fold the cover small enough to get it back in the bag !!
If the garage is damp or cold the cotton / fabric based covers will hold moisture against the paint which has led to local micro blistering on the MG. In winter I now use large non fitted breathable outdoor covers which lets the air circulate or simply don't bother, the dust will wash off in the spring.
To be honest the cheapest and best option is to throw an old bed sheet over the car in the summer months and not bother at all in the winter.
The covers I use have been supplied by http://www.specialisedcarcovers.com/spe ... covers.htm
