adding thinners will give the paint less 'body' but application will go further.. will also give you runs and drips in your paint if your heavy handed.... Looks the goods so far mate!! its not a race so take your time and do it right the first time eh.
IF you do want to mix in some thinners.. be subtle with the amount you use.. presuming your mixing and painting out of another tin and not the primer tin?? if so.. pour aprox 4 inches of primer into your pot, and aprox 1/2 inch of thinners.. stir though thoughrouly(sp?) if its still to thin you can use the non thinned primer to thicken up a little at a time is better than to much!
Keep up the good work mate! its all about learning!
Rivet or tech screw that wind guard back on to!!