I'd be doing that as a DIYer too.
In terms of the rust treatment, what seems to work for me is sanding it with something like a small flap wheel to get the top rust and loose flaky paint off so you have a small area of shiny metal around the pitted area, then using something like the green acidic rust dissolving gel to remove all of the rust (be prepared to keep repeating that step, washing off every couple of hours or so, drying and repeating, keeping the gel 'wet' rather than leaving it to dry out. You will ow when you have done enough as there won't be any black bits at the bottom of the pitted area. Then wash well with clean water and (important) dry it very well, heating the panel is a neat way of ensuring it is properly dry and you need to be sure that all the acid gel has gone too.
Primer of choice for this would be that 'bonda primer' that replaced red oxide for use on classic cars and structural steelwork etc. it's an ugly red colour but is chemical formulated to go onto bare metal, but unlike most other primers, is waterproof and effectively seals the pitted area and stops it from rusting much better than anything else I have seen. Brush that on, leave to dry, brush more on, leave to dry, brush more on, then leave for several weeks to fully lose all of its solvents. Your car will look a little scruffy during this time, but that patience will be worth it.
Then flat it back with a fine wet and dry paper (P400 to start with) and then P800 to get rid of any high spots in the primer, brush marks or sanding lines and it will be time for more conventional primer. I would spray that, overlapping a tiny bit into the painted area around and wait patiently for that to be thoroughly dry (more than a day even in this weather) before flitting that back.
Read about guide coats and decide whether you want a really good finish or just something 'not rusty' and either go through the guide coat process to get the panel really accurately flat and smooth, or move on to paint. The one thing I would say is that the time spent in prep is far more important than rushing to get paint over the top and trying to fix it later with polish.
I have had good results by stripping all the wax polish off the panel to be sprayed (G3 polish and some more patience) and then working my way around the area to be sprayed with some P1200 paper to gently provide a keyed surface to help the paint to stick - wax contamination is heart breaking and peeling paint later is similarly distressing if you have spent some time on it.
As far as colour goes, if you can get a fairly well matched aerosol and the car is a very dark colour, it is probably just worth doing that. I'm not sue if Holts still deal in their "spray match" paints but some paint factors will put professional quality paints into aerosols for you to use at home and that's probably the best way to get a colour match. The decision you have is whether you want to paint the whole panel or just a small part of it, a poor match might look awful on a spot, but you might get away with covering the primed area and overlapping your spray slightly outside over the adjacent paint getting progressively thinner to allow a soft blend to form, almost dust looking at its edges. Let the colour coat(s) flash off (get 'touch dry') and then go over with lacquer (I'm assuming your car is a metallic).
The lacquer coat should go much further than the colour coated area and I'd say do the whole panel if you can, as then you won't get any hideous spray lines to rub out (and inevitably chase around the panel).
Once you've put the clear (lacquer) coat on, leave it well alone for at least a fortnight until you can't smell the solvents in the paint and you can't make any kind of mark in the new paint with your finger nail, then start to polish it. If you're the king of the spray can, some cans actually give a very good finish, but more likely you might find you want to start with a very fine wet and dry paper (P1200 perhaps) and go over the whole panel to remove any roughness in the lacquer. Do this 'wet' as the water lubricates and cleans the paper and stops it from clogging and scratching the paint. Then go over with G3 rubbing compound and finally your polish of choice.
That might be a bit OTT for you and I haven't covered everything in lots of detail - be prepared to be doing bits to it over a few weekends and open up a few cans of patience. The waiting times are based on my own experiences as the paint tends to shrink as it dries and where you might rub it back to a flat surface and then paint over that, as it continues to dry it shrinks more and looks bad later on. You can avoid that if you are expecting it.
Or, go as far as the 'bonda primer' stage, carefully flat that back and (eek) brush on some colour and lacquer. Roughly right colour, roughly flat and rust free panel...
Hope this helps, I'm sure I will have missed something but someone will correct it I'm sure. (Please do!)
Ian.
