If you have glassed wood surfaces, you are far better off brushing the first fill coats of primer. It takes far less primer and you can fill the low spots and any pinholes much easier.

As for thinning, it depends on which primer you are using. K36 can be thinned about 20% for brushing. I use MP182 primer and 10% thinner is all you need.

And always use the fastest drying thinners in primer.