Is sandblasting sufficient for bond prep of composites?

[Dan Stout]

Hello composites gurus!  I am always looking for ways to cut time and manual labor in my garage composite projects.  Most of the parts that I build are wet layup in a mold, then hand sanded and trimmed before applying a final coat of epoxy resin.  Recently I had access to a sandblaster and used this to rough up my molded parts. When I applied the top coat of resin, it did not evenly coat the part, but instead beaded up.  Should sandblasting be sufficient for bond prepping composites, or for applying a layer of resin like I do?

I wish I had taken pictures of the sandblasted parts and the resin beading up, but I didn't. 

[John Kimball]

Dan,

As you have come to hear me say too many times: “it depends”. 

Sandblasting is an acceptable way to prep a surface for bonding, but is only the first step. proper cleaning is just as important.

Also you consider what blast media you use. For metal, sand is excellent as it won’t damage the metal too severely, but can if you blast too long. With composites, it is plastic and much softer than sand, so it can blast right through the resin and fibers quite quickly. for softer materials like plastic and resin, you can use media like walnut shell or baking soda (you can get a 5 lb bag at Costco). These should pit the surface, but not erode it like sand would. But you will still need to clean it aggressively to get any embedded particles out. You can use a stiff brush to help clean it with IPA.

[Dan Stout]

Ok.  I thought I did do a thorough cleaning, but it was just with a cloth and IPA and Acetone.  I had no idea I would need a stiff brush to properly clean it. 

I was assuming you were going to ask what kind of blast media I used.  I have no idea. lol.  I will have to look into that.  This was my first time using any kind of grit blasting system.  It quickly removed any shininess and made the surface look dull, but it didn't do much more than that.  If I held it over a specific spot, I expected it to start breaking into the fibers like you said, but nothing happened.