Painting the bottom plastic trim

I still wanted mine darker but with a little bit of color spereration so I painted mine satin black rather than going with the original charcoal gray. I used 3M rockguard which is very easy to apply and holds up extremely well. It also has very good flexing charistics as opposed to paint so I won't get the cracks that I have seen with the color matched lower valences and trim.

Doesn't the rock guard have a very course sandpaper-like finish to it or are there different types of rock guard?

It depends on the amount applied with the 3M product. If you want a rougher finish spray several light dusting coats, for a smoother look spray it on fairly thick. You can also take a wet lint free cloth after it has dried and scrub it smooth too. I found this out after a trip to the track. I had alot of rubber built up on the lower trim and after scrubbing it off, gave it a nice very slick satin finish. Best of all, if you get some scratches (or in my case buffing marks) on the lower trim, you do not have to blend it in if you deciede to recoat small areas.

how much paint (of the actual color alone) do you think is needed? i plan on buying from here, unless you know of any other places to get it for cheaper?

http://www.automotivetouchup.com/choosecolor/buypaint.aspx?year=1998&make=Buick&model=Riviera&colordesc=Silvermist+Met%2C+WA301D%2C+No%2C+No%2C+*52964

a quart should be enough. as long as you have a HVLP sprayer.

maybe 200g of paint would be enogh...
after painting, glossing and so on...anyways when it's done then apply m3 guard tapes to place what may get hit whit tiny rocks and dirt and it should hold on for years. when the tape goes bad just change it and it looks like new again.