Advice reg sanding before primer

[TROOPER88]

Hi
I am restoring a mk1 Golf GTi and thanks to some advice from fellow Pistonheaders I have just purchased a good sander.

The shell is bare and was painted a Baby Blue colour a couple of years ago and it seems the paint is quite thick.

I was advised by a friend at the local bodyshop to go over the whole shell with 180 grit discs, then 320 and then it can be primed.

I just started with the 180 on the roof and need advice as to:

1) How much hand pressure should I apply through the sander? It is a rotary palm type
2? How many passes would be good enough? If this is to difficult a question to answer, how long should it take to sand the roof on a small car such as the Golf?

I guess the aim is to remove any gloss from the surface to give a good key for the primer?

To do the roof with the 180 only took 5 minutes. Naturally this is only the roof but I do not see why I could not sand the whole shell with 180 in a couple of hours, the same again with the 320 before priming? Or am I missing something here.....

Many thanks if anyone is able to offer guidance.

[theshrew]

It depends mate.

Personally if it was me doing this id take all the paint off. Its a old car bound to be some body repairs on it. If you go back to bare metal at least then you can tell if they have been done correctly and if not do it yourself before painting.

Anyway if you dont want to do that the simple point is to remember the key to a good paint finnish is the prep work. A 5min blast over a roof wont be to great.

[S2Mike]

Its one of those "how long is a piece of string" things.
Sand until the finish is smooth, the fiddly bits by hand in the grooves and curved areas will take considerably longer, around door edges etc, plus getting rid of any polish, grease, dust will have to be done before any painting or you will ruin the smooth finish, or it will lift or peel. You can easily spend 4 or 5 hours sanding, running a hand over the whole car to feel for rough edges and ripples in the bodywork. 180 will be fine before primer, then use 600 wet and dry on a rubber block to flat the primer, all over. Then depending how good you want the finish to be, I would put one coat of the top finish paint then rub it all down again with 800 wet and dry,this will highlight any reaction of the paint to what is already on the car, before finally getting the top coat on for real. Then you have many, many hours with polishing compound T-cut and polish before its done.

[TROOPER88]

Thanks so far chaps.

To be honest the shell is extremely straight and I do not fancy taking it back to metal. I have sprayed a car before and was very pleased with the results. The difference this time is that last time the shell was new and only had a couple of coats of factory primer on it.

I will really be starting this on Monday and Tuesday so I think I will go over everything with 180 on Monday and 320 ish on the Tuesday. This will give me many hours to ensure I am happy.

Thanks