Morris Minor Owners Club

It isn't always absolutely necessary and i know some places don't use it to save a couple of quid, however the places that do GOOD resprays will use it.
If the car isn't rusty, then can it just be polished back up (using a commercial polisher/buffer and a lot of cutting compound)?

If it is sanded it is more likely to go through various layers of paints - that can show through a topcoat if if doesn't have enough nice flat primer underneath.

No it isn't always necessary to apply undercoat, the important
thing is to have a completely 'flat' sound surface to which you can
apply the new top coat. By this I mean that there must be NO
trace of the original shine left at all. This is the only way to ensure
you have removed all traces of polish/silicones/ and any other
contamination which would prevent the new paint from adhereing
properly. A further complication in your case is that you could
well have a car which has synthetic enamel as the factory applied
top coat....which may or may not accept a new celllulose top coat.
For example your chassis No, is likely to be FAA11/..... the last
figure one would indicate Synthetic paint, if the last figure is a
3 then it is cellulose. The only way to see if there will be no
reaction between the paints is to test spray a small area and
pray! Would be a lot easier if you try Rays' suggestion
and try T cut first.

primer is used on bare steel, to stop rust.
also primer has filler in it to help with flat surface, use a block when sanding.
if you have top coat on car, rubbing down with red scotchbright, will give a good key for new paint, work it good. or 800 wet an dry with water and lots washing up liquid bubbles on paper, its the soap that does the cutting not the paper. use window cleaning squige to clean as you go

reactions are caused by wax as stated before. lots of soap. somtimes in celle put i drop of silicon oil can help stop it. watch out if the car has been sprayed more recently in 2 pac. only 2 pac can be used unless you coat entire car in special primer if you want to use celle

its the soap that does the cutting not the paper.

I always thought that soap was more of a lubricant rather than a cutting agent, it just shows you learn something new every day.

And I always thought that soap was used to stop the paper clogging up!

I thought the soap was a lubricant and the water stopped the clogging.........ha ha! Whichever way it seems to work!

brixton, If you have this info, could you enlighten me on water based paint for vehicles, can this be used over a cellulose primer? Also can 2 pac be sprayed over cellulose primer? I have a van that I have sprayed with primer and before carrying on with cellulose top coat, was wondering what my options were

yes les it does help the paper cloging up, but it realy does cut better with lots of bubbles.
i dont realy know much about water based paint, except that the powers that be are trying to get all the spray shops to use it. i get the felling that it will go on to celly primer, problems arrise when one trys to put celly on to 2 pac. nothing compares to a cellious polished minor
paint built up on paper will cause scratches. when the top coat is dry, cut it back with 1200 grade wet and dry, and again loads of fairy liquid no cheap brands of washing up stuff only fairy. use a rubber block, and the palm of your hand not fingers. the paint supplies will tell you about water/2 pac. go for cel . very few times have i had a reaction from paint, usually due to wax. sometimes on late 2 door saloons the roofs cause some prob.