Okay,
As a man of my word, I have painted my R100 with the old rustoleum, using a paint brush. Here is a picture of my progress. It not completely finished yet as it still needs a bit more buffing rubbing down. I was just trying the tank for size last night.
I could be riding the bike this weekend - weather permitting.
Brief Report on the £20 paint job:
1. I used a brush and not a roller - too many compound curves etc, especially on the tank.
2. The paint needed to go on fairly quickly, and you do not get that much time to mess around with it. Although it is plenty of time if you think about what you are going and work methodically (I am thinking about painting the top, bottom and sides of the tank here).
3. Do not be tempted to go back to the paint when it starts to go tacky - it will just accentuate the brush marks.
4. I did not thin paint at all - painted straight from the tin with a quality brush. However thining (up to 10%) might make application easier - just expect to add another coat.
5. Runs are you enemy - should be avoided at all costs (although it will happen until you get your 'eye' in). When rubbing down a run, be ready to find underlying paint still tacky. Put the job down for a day and let the uncovered paint harden before continuing - especially if it is the final rub down.
6. No primer used. Paint went straight onto metal/filler/old paint. It is self priming. I had 3 or 4 dents to fill on the tank before I started.
7. I painted it the garage/office over the Christmas break, the temp was hovering around freezing outside - I recoated every day in temps of 10 degree C in garage/office. With aprox 20 hours to harden/cure between coats.
8. 4 coats should see you right with a brush, I would say, with a bit of rubbing down between coats to remove dust particles and high points, etc. As Rob Valdez indicated, change direction of the brush stroke on every coat so as not to accentuate brush marks on previous coats. A good thick layer of paint to allow for the final rubbing down process.
9. The paint is easy to rub down, using mainly 600 grit wet and dry, but do not get too carried away, if you go all the way through, or find a subtle dent that needs filling (like I did) you need an absolute minimum of two coats (with a brush) to cover it up - think of the final coat as being used to fill in the imperfections of the previous coats, and you will lose the vast majority of it during the final rubbing down. I finished off with some 1200 grit (without intermediate grades) before I got started with a bit of t-cut and then polish. With 4 coats applied with a brush, there should be plenty of depth though - which is why I was able to do most of the work with the 600 grit.
10. The paint is 'low odour', though not 'no odour', so I did not get many complaints with regards to smell. My garage is integral to the house, so it was wafting through as I entered/exited the garage. But it was not really a problem.
11. Mess - well not much. My garage is not orange - and there is no overspray anywhere. I have to wipe down the area I was doing the rubbing down in - and that is all. I was polishing the tank in my sitting room last night whilst watching the goggle box...
So there you have it. I am not going to say it is a perfect job - I would say it currently passes the 6 inch test, but that is only because I am lazy - Monte would do a better job of the finishing, I am sure.
As I still have some work to do, and will probably go back to the 1200 grit, one lazy afternoon in the spring/summer, before polishing up again. That way I will be able let the paint harden further - although it does not really seem that soft.
Oh, and I was quoted £400 for this paint job - so that £380 in the bank for me and I still have enough paint to do it again another 2 or 3 times!

Paint colours are limited - forget metalics. Some colours will take more effort to get that perfect finish than others. Gloss black comes to mind. But it is attainable with a little effort.
I would not recommend this for you concours R90S, but I would recomend this for your daily rider/hack if you are happy with the non-mettallic issue. You, can of course, mix the paints, so if the available colours are not quite right, just mix...
Hope this helps
Steve H