I am currently painting a mk1 mini, I tried cellulose high build upol p88 and as long as you have a big enough tip, I used a 2.0mm, then you can spray it with minimal thinners, it builds ok and sands easily. If you are not using a high build primer then this should prevent the problems you are having, then flat this with a guide coat to ensure perfectly flat.
I however decided to change to 2k high build and use 2k direct gloss as its easier to work with and doesn't appear to react with anything, also gives superb gloss and flat finish straight from the gun. I used a gerson mask with the primer but was never 100% confident so bought an airfed mask with correct filters as I have more than one job to do.
I also tried the non ISO 2k but the primer is not easy to sand and is not thinners resistant as it doesn't cure properly, the non ISO hardener only converts it into an air drying single pack paint, the same product is used in aerosols.
I would either stick with cellulose or use proper 2k but if you do use 2k then use an airfed, they are brilliant and give you the confidence to stay in the garage long enough to do a proper job.