I see lots of posts telling people what grit to use for different(There is a super good sticky on here about it too)

But apart from sanding at the right grit, what are the signs that your paint is not going to adhere properly, its ok it looking ok when you spray, but are there any tell tail signs as you are spraying that you are going to have problems down the line that you can fix straight away.

Example, I was spraying on metal the other day, I had a black wing, I used 400 grit and sanded it all down,(not taking off the existing clear, just giving it a nice bed for the base to sit on)

I know 400 is a bit deep but I really wanted to make sure I would get a good grip, but I noticed tiny spots where the paint just didn't exist appearing, kind of like spots appearing, (I wish I took a picture at the time) is this a sign of adhesion problem? or is this something else?

i suppose there is really no way to tell buy looking for sure if the paint is going to stick. BUT as a painter you should know that the right surface prep and products is going to do the job. when i spray something i know its right as i prepped it right. your surface should have some profile to it. i always say NO SHINNY SPOTS. what grit you use is up to you, but there's somewhat rules to follow. you don't want to spray base coat over 180 profile. your spots sound like fisheyes or possibly pin holes. cleaning and grit profile are very important, its the base for your paint job get it wrong and no matter what you do after its going to probably fail. there's lots of info in the sticky's above on sanding and prepping. post a picture
Jay D.

I think pin holes sound like the cause of this one now I've googled what they are.

Just look like spots of paint that are missing, and pin holes seem like the cause, but how I would fix these in future I don't know, seem to be caused by cavitys, I inspected the paint work before applying anything to it and it looked flawless, obviously not that good as I thought! ha

if you didn't see any pin holes during your inspection then that would ALMOST rule that out, I'm still thinking fisheyes. what did you do to the surface before you sprayed your paint. did you clean it good with wax & grease remover. post what you did for prep work. pictures would really help PICTURES, GOOD PICTURES, PICTURES,PICTURES.
Jay D.

You was right it was pinholes, my inspection was poor

I stripped the paint off, and I could see the smallest holes where the pin holes appeared, I couldn't even feel them with my fingers, but I could see them fill when I ran some body filler over them and it fixed my issue! also a bit less paint helped aswell to flow into any cavities if there were any left

i like a happy ending
Jay D.

don't we all