snap fit to test...then remove to paint. parts are painted piece by piece...INCLUSIVE of top coating. u can go the easy way out...top coat the gloss or flat after finish assembling. but that's the easy way out. wat u shld do is instead top coat it by parts or sections before final fixing.

alternatively, pre-plan ur part colors. then start painting before snapping things in place. and if necessary, at times...yes, u'll need to mask to spray paint 2 parts that are in diff color. especially if that part need to have the seamlines covered.

anyway, this is my standard procedure...
1) cut parts fr sprue
2) trim the extra nips and sand them
3) read manual...look out for seamlines to cover (usually do this at beginning before start cutting lar...but what the heck. do now oso can)
4) fix parts that need to be glued (to cover seamlines)
5) putty the seamlines and wait for putty to dry
6) sand the putty to remove seamlines
7) spray surfacer to check for errors, repeat step 5 n 6 til seamline cannot be seen
8) paint internals (i know some ppl skip this...or paint certain internals only... )
9) paint external armor, etc... color scheme should be planned before reaching here.

ok...here, the process will defer depending on whether u're using waterslide decals or dry transfers (those that come with MG kits) or stickers. But i dun use dry transfers...only use waterslide...so the following is catered for that. for dry transfers or stickers...just top coat after putting the sticker/transfers on after step 9.
10) gloss coat the external armor
11) put on decals
12) gloss coat
13) panel line and do an ink wash on the kit (some ppl do this as the end)
14) flat coat the parts to ur personal liking
15) weathering of kit using pastels, enamels, etc (up to personal preference)

note, the above is standard process to built n paint a kit. excludes extras such as modding for articulation, added detailing or scribing of panel lines. those...u can do after step 3...by doing snap fitting n deciding how u want to mod, scribe panel lines, etc. for snap fitting...cut the joints a bit...to ease takin apart the snap fitted parts. ok..tat's a lengthy explanation. i think it can be found somewhere in this thread..but wat the heck...typed already.