(I hate it when the fast typists get their posts in so quick. There was nothing here when I started to type)
Wolfpac… welcome to the forum. I only build aircraft but most of the tings you ask abut apply to just about all subject matter.
Creases - What to you mean by creases? There are 2 kinds in lines on a part… one is the mold line made be the 2 halves of the mold together and the other is a seam… when joining to pieces. For mold lines I will lightly scrape the line with a sharp Exacto blade then a light buff with a green Scotch Brite scouring pad. Seams require a bit more attention… sometimes. There are many different techniques that can be used. If the part has good fit and you use a “welder” like Tenax, a little squeezing will push some of the softened plastic out of the seam and when that has hardened, a light filing or sanding stick will usually do the trick. If there is still a gap you will need some filler… Squadron, Tamiya, Mr. Surfacer and so on.
Decals – The white part you mention for the carrier film sounds like silvering. Apply decal to a gloss surface. A gloss coat of Future or other gloss coating will give a smooth surface that will not allow and air under the decal. That is what causes the silvering. A top coat to seal and the clear bits should almost disappear. For the curve surfaces there are many products t help the decal snuggle down and conform to the shape. Micro Sol, Micro Set, Solvaset to name a few.
Photo Etch (PE) – Size and complexity of the part may determine best folding methods. I use a small pair of squared, smooth pliers for most. Larger or longer pieces and be taped to some glass of a smooth ceramic tile, a metal ruler of straight edge held along the line to be folded, then carefully slide a single edge razor under the part and fold it up. There are after market (AM) tools to do that too such as an Etch-Mate. I use super glue, usually applied with a tooth pick to the part and tweezers to place the PE part in place. I made a small tool to apply tiny drop of accelerator if needed. I tool a sewing needle, cut off the end of the eye, making a very tine little 2 prong fork. I mounted it in a wood dowel handle and use it to dip into the accelerator and apply to the superglued part to get it to set instantly. And yes… sometimes you need 4 hands.
I will defer on the primer question to someone that uses acrylics since I only use enamels.
The link below is to swanny/s site that has lots of tips and technique. Do a search in the forum of some of the individual things I mention and that should give you plenty to go on.
http://www.swannysmodels.com/Tools.html
And very nice work on the bikes.