Using Dry Powder Guide Coat

[dhutton01]

You do not want to let any build primer sit exposed for very long because they absorb contaminants quite easily.

The SPI Tech Manual states:

Sanding: Depending on how you applied it and current temperature this primer should be ready to sand in 45-60 minutes. If sanded primer has set more than 24 hours you must scuff first with a gray scuff pad or equivalent before applying another product.

Maybe define “very long”. It can go a few weeks or more in a clean shop, especially unsanded imo. Months and years no…

 

[Chris_Hamilton]

As long as you have epoxy over bare metal, it doesn't matter how long urethane primer is on there. Where one would have issues is if no epoxy was used on bare metal, say only either etch or urethane, then it sat for a long time. The urethane primers can and will absorb moisture and cause the metal to rust.

As for the 24 hour rule, all the manufacturers used to say that in their TDS. SPI is one of the few who still do (good thing). Because urethane has quite a bit of solvent in it it can self heal if sanded shortly after application then left to sit. The self healing is most pronounced in the hours/days after application. But some solvent can remain in urethane primers longer so it's good practice to do the 24 hour rule.

A quick way to get around it is to use sanding paste and a grey or maroon scotchbrite pad and go over it as a last step prior to masking. It will scuff the panel and ensure adequate scratches are on the panel before applying sealer..

 

[hapcj7]

Maybe define “very long”. It can go a few weeks or more in a clean shop, especially unsanded imo. Months and years no…

And to add- If it does go a few weeks, can it be scuffed with grey pad or equivalent, then base immediately thereafter?..

 

[Chris_Hamilton]

And to add- If it does go a few weeks, can it be scuffed with grey pad or equivalent, then base immediately thereafter?..

Yes that is what I was trying to describe above. If you do it pad only you are better off using a maroon scotchbrite and going over it throughly but with light hand pressure. Gray doesn't really scuff well without the addition of sanding paste. You would also be wise to seal before going to base to give you better adhesion.