Fiberglass to primer bond??

[dwilson]

I am building a 9A. I fit the plexiglass to the tip up frame. Got a good fit too!

I am about to fiberglass the two together when I realized -- I already primered the frame with 2 part PPG epoxy primer.

So-- what do I need to do to the primer to make the glass stick??

Thanks,
Duane

[Highflight]

Are you saying that you intend to use fiberglass to bond the canopy to the canopy frame?

Or do I misunderstand?

[dwilson]

No. I am worried about the forward edge of the canopy where it meets the aluminum frame.

Will the fiberglass fill between the plexiglass and the aluminum stick to the primer, or do I need to sand off or remove chemically the primer?

Sorry for the misleading first post.

Duane

[Highflight]

I assume you're referring to where the front canopy is molded into the aluminum top panel.

No, your best bond will be had by sanding the primer off the aluminum and roughing it up with something around 80 grit on a DA sander. The epoxy resin (e.g. West Systems) will adhere best to the bare, sanded aluminum.

Be extra careful that the aluminum is clean with no oil, handprint, or silicone contaminants on it.

[rv72004]

I beg to differ, but when I did some experiments, I found epoxy to adhere better to my primer [strontium chromate] than it did to rough bare aluminium.A lot better as well.

Your milage may differ with the primer you use, but I still believe thats what a primer is for : to stick a top coat to a substrate.

Do some sample tests and youll see what I mean.

[Highflight]

Originally Posted by rv72004

I beg to differ, but when I did some experiments, I found epoxy to adhere better to my primer [strontium chromate] than it did to rough bare aluminium.A lot better as well.

Your milage may differ with the primer you use, but I still believe thats what a primer is for : to stick a top coat to a substrate.

Do some sample tests and youll see what I mean.

The problem is, epoxy resin has no chemical bonding characteristics while a primer such as strontium chromate is specifically used to work chemically. In the case of strontium chromate, it's primary purpose is for corrosion protection. It is also an adhesion promoter, but it achieves that by it's chemical properties that react with the topcoat put over it (and again, epoxy resins don't "care" about that).
If you doubt that, note that there is a time window in which a top coat MUST be applied, or you have to rough up the strontium chromate and start over.
What this means is that strontium chromate "case hardens" beyond it's cure window and anything put on top of it after that may in fact result in less adhesion than if you had left the aluminum bare.

That leaves you with trying to get the best physical bond you can get which is achieved by scratching up the surface of the aluminum really well and making sure that there is no reason to weaken the bond through contamination.

Epoxy resin will seem to stick well to just about everything, but I have doubts that strontium chromate will make the bond better between the epoxy and aluminum.

[az_gila]

Originally Posted by Highflight

Epoxy resin will seem to stick well to just about everything, but I have doubts that strontium chromate will make the bond better between the epoxy and aluminum.

This might not be true if it's an epoxy-based strontium chromate primer.. like the AKZO stuff...
http://www.aircraftspruce.com/catalo...akzoprimer.php
or the DP equivalent (DP-70?)

gil in Tucson