I've ruin my figurines with an undercoat by a too hot sun... so now they are grainy.

Did you have any tip to fix it ?

Simple Green to strip and start over is my recommendation

Or strip it with Dettol if youre from the UK or neighbouring countries. Theres a knack to it (happy to elaborate) but it works. Had the same issue with my Luke but he's fine now

Before going to the trouble of stripping you could try gently scrubbing it with a cheap toothbrush.

I had a mini in a similar state (I might have held the can too far away) and that took off the powdery texture but still left a reasonably smooth coat.

Then, if you're not happy with the resulting surface you can proceed with stripping.

I think the suggestion to strip it (with whatever appropriate solvent is available - just be careful) is a good one. However it depends on just how bad they are. I assume by the word “ruined” that they are pretty bad.

The truth is that with rattle can primers you have a lot o variables that are really hard to control. Weather (temp and humidity), the quality of the primer (which in my hands varies a lot from can to can), how clean the spray nozzle is, the distance you are spraying, the amount you are spraying etc. can all have an impact. If you spray from too far you tend to get rough textures and spotting and if you spray from too close you tend to overspray and lose detail - at least in my hands. It’s tough.

So two things:

1: Have appropriate expectations. You aren’t likely to rattle can a squad if 7 and have them all be perfect. At least I never could.

2: I know no one likes expensive, space gobbling suggestions, but if the quality of the primer coat is really bothering you, I solved this problem with an airbrush. I originally got one to prime because I was worried about temperature and humidity. But in using it I have found that even on my worst day, and with my sub $100 airbrush the ability to deliver a nice, even primer coat is vastly superior to a rattle can. The ability to have lots of control over the airflow and the amount of paint being delivered let’s you prime in tiny, even layers and the airbrush primers perform in a way that is generally very forgiving.

What everyone else said about stripping. Plus the following future advice. Always have a piece of cardboard around when spray painting miniatures. Test the spray on that and see how the paint is reacting to the weather, how distance effects the paint, etc. Every can, even the color/brand, could be different and even the same can might not be the same day to day.

I always spray prime minis on an old pizza box and test my paint out on a corner of that.