Scale 75 polyurethane primer, a quality primer and easy to use straight from the bottle.
The Madrid-based company made a high strength polyurethane primer, more robust than lesser-quality products. The result is a more robust base for your minis; plastic metal models can survive scratches better until paint and varnish are applied. Expected quality from a product between history and legend, paint pigments are consistently finely ground.
The first tip, shake the primer until your arm falls off. Especially if the bottle sat for a while, the high-strength binder settles. A couple of stainless-steel balls are required.
Once shaken, primer can be used from the bottle; no thinning is required. Thinning any more than 10% would make it runny.
The second tip, use high psi compared to other primers. For example, Bone and Ice charm sprayed at 35psi, and Holy charm at 40psi. Most painters used a 0.35 or 0.4 needle, because 0.3 would clog too much, and 0.5 would be too wide to atomize the primer.
Instant Colors primer and Surface primer have the same formulation, and the three Instant Colors Charm primers make up a system that build a nice neutral white and also a nice warm to cool gradient, straight out the bottle! The results is a great base for contrast painting or airbrushing from light to darks.