Primer Colors?

On the forums, there is always questions about primers.

From an internet search:

"Primer allows the final color coats to adhere better. Primer is designed to adhere to surfaces and to form a binding layer that is better prepared to receive the paint."

I've had the best results using Duplicolor Filler/Primer shown at the right. 
It's available at auto supply stores.

This Filler/Primer does two things, fills imperfections (after sanding) and primes the surface before a white undercoat.

From the Duplicolor website:

Filler Primer - Dupli-Color Filler Primer features a hi-build automotive formulation capable of filling deep scratches and minor surface imperfections. A fast-drying, sandable finish allows for easy feather edging while providing a smooth and rust resistant surface that promotes top coat uniformity.

• Improves top coat adhesion
• High build formula
• Easy sanding properties
• Dry to touch in 30 minutes
• Handle in 1 hour

This Filler Primer does a great job of "follow-up" filling of any remaining body tube seams and balsa grain.

After fine sanding the gray primer down, a white undercoat always precedes any color coats.

So, one way to go - After filling the balsa and body tube seams - 

Use GRAY primer/filler first then follow with a white undercoat (or white primer) to make the final color coats brighter. 

Most primers are used for car finishes, where a large area gets a single overall color.

BLACK primers can be used if the overall rocket color is black. If you use a black primer, fewer black final coats will be needed. 

RED primers are heavy fillers, made to fill scratches on car finishes, not normally for hobby use. Many builders use red Bondo Spot Filler on body tube seams with good results.

GREEN primers are used on bare metal to prevent rust.