Code requires that for all new work DWV plumbing pipes must be installed in a manner so that the inspector can read the MFG writing on the pipe to insure all DWV lines were installed with sched. 40 pipe. Once the pipes have passed inspection there is nothing in the codes that would prohibit painting the pipe. In fact, code recommends painting any PVC pipe that is exposed to sunlight, such as but not limited to vent stacks above the roof, with Latex paint to reduce deterioration from UV light

I figure after demo part I’d just need to do the insulation between rafters.. then cover with drywall, plaster, paint.. ? Have you been to the attic? Got any wiring up there? Any ductwork? Any plumbing vents? What is your plan to vent the roof? How are you going to insulate? What R value will you have? How do you plan to brace the trusses? What about climate control--HVAC? You will be adding a lot of cubic feet of new space that, in all likelihood, your HVAC system isn't designed to handle. Not only additional cubic feet but also the problem of stratification. How & to what do you plan to fasten the new drywall? You can't fasten drywall into the side of the truss; you will have to install some nailers/ ledgers to fasten drywall to. Trusses are typically constructed of 2 x 4's. Of particular note, the top chord is a 2x4, meaning there will be 3.5" between the roof deck and the bottom of that chord. Which will be buried in insulation. So, you need to realize that you will not see that top chord from the room below. What is you plan to deal with/ cover the gaps in the drywall caused by the trusses themselves? IOW's, what you want to do is a good deal more complicated and labor intensive than what I *think* you are thinking it will be.

Congratulations on your project. You asked specificially about ceiling treatment, but there are also a host of other issues to convert an "exterior" space to a habitable one. Here's some thoughts about garage conversions: Floor slab: Is it solid with no cracks? Is it at house finish floor level or are you OK with a step down? Is it insulated below the slab and on stem walls? Are you in a climatic zone where winter coldness is a factor for the concrete slab? If so, floating a floor on sleepers (with insulation) above the top of the slab is an option. Walls: Will additional sealing of cracks and insulation be needed? Will new gyp board and trim be needed in some areas? Ceiling and roof: What sort of roof structure do you have? How will you insulate and vent the roof to prevent heat loss and moisture condensation/mold? You will likely be better served to have some sort of seperation between the habitable space below and the insulation/moisture barrier above which is usually not very visually attractive (what a ceiling usually does). Although I hate the look of a suspended ceiling, it may offer one inexpensive option for a new ceiling if the existing roof joists or trusses have uneven bottoms. Systems: How will you heat and cool the new space? Will you need to run new HVAC ducts, control wiring etc.? Will you need new electrical circuits, controls and a secondary panel? Need water and drainage? I'm sure you have thought through all of this, so pardon me if this is redundant. Good luck!

You could replace the batt insulation with spray on foam that is paintable, but that might not be cheap either.