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I'm running into a problem with sandblast mask pulling up primer from HDU. So far I have used two grades of Anchor mask (128 medium tack) which is advertised for pre-coated HDU. I'm using FSC 360-WB knife-grade filler to skim coat HDU; it gives a nice slick finish, but after blasting the mask pulls it right off. I don't topcoat before I apply the mask, just the skim/primer. The mask doesn't pull up primer when weeding before blasting, but afterwords it pulls it right off. Suggestions?
------------------ "A wise man concerns himself with the truth, not with what people believe." - Aristotle
When the going gets weird, the weird turn pro. - Raoul Duke (Hunter S. Thompson)
Cam Finest Kind Signs 256 S. Broad St. Pawcatuck, Ct. 06379 "Award winning Signs since 1988"
Posts: 3051 | From: Pawcatuck,Connecticut USA | Registered: Nov 1998
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Hey Cam, We have had this problem before. You could use SignLife's "First Step" but it doesn't fill the cell structure as well as "white primers" do. Here's what we do with HDU: 1. Prime with three coats of Jay Cooke's primer. 2 Let dry thoroughly 3. topcoat with acrylic latex enamels (Sherwynn Williams) or bulletin colors. We prefer the former. 3. Let dry/cure for one week.(very important) 4. use Anchor #153 medium tack stencil. 5 blast with LIGHT PRESSURE(most important). try about 40 psi and KEEP THE NOZZLE MOVING RAPIDLY(next most important). Works! No more peel-up!
I've read that someone paints the waterbased acrylics directly onto the raw HDU without primer. I think it was Gary Anderson. We haven't tried it yet though. E-mail me if you need any help.
------------------ Wayne Webb Webb Sign Studio,Inc. creators of "woodesigns" "autograph your work with excellence" webbsignstudio@digitalexp.com
[This message has been edited by Wayne Webb (edited August 15, 2000).]
Posts: 7404 | From: Chipley,Florida,United States | Registered: Oct 1999
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Cam, we have had good luck with Anchor #153 and Really like the thin Hartco, not sure of the number. The Hartco works great on HDU, tracks well and cuts fast on plotter, sticks to primer but dosent pull it up, and the cost is half of Anchor #153
I just finished priming a piece of 15 HDU (PB) with Chromatic High Build Primer, which I helped develop and is now part of the One Shot family. Just one coat and light wet sand with 600 grit. Smoooth as a baby's...and I have never had a problem with adhesion/pull up. Mask adheres nicely to this finish too. Most of the acrylics adhere well to the urethane and are not subject to the heat from blasting. They also have good intercoat adhesion with most any topcoat. Any paint will adhere acceptably well directly to the urethane foams, hence the use of the pigmented acrylics direct. The reason for the primer is to fill the porosity and to ensure durability of the topcoats as well as their brilliance over a white primer. Solvent based primers and fillers have been problematic for me as they have intermittent adhesion due to cratering during drying and curing and the waterbourne acrylics just work better.
------------------ Kent Smith Smith Sign Studio Greeley, Colorado, USA kent@smithsignstudio.com
Posts: 1025 | From: Estes Park, CO | Registered: Nov 1998
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Cam, I agree with all the above and would like to add one more thing. My theory is that the pressure or velocity of the sand particles has a lot to do with the peel-up problem. We have found, on a couple of HDU blanks after blasting, small craters indenting the paint. I was puzzled at first because I knew that the surface had been sanded perfectly. These were caused, I believe, by extra large sand particles. When these are propelled against the stencil at high velocity (high pressure) they overcome the resilience capacity of the rubber stencil and dent the paint/substrate underneath. I believe that medium sand, when using enough pressure/velocity to overcome the protection of the mask, pulverizes the paint, primer enough to break the bond with the substrate. Kind of like wearing a "bullet proof" vest. If you're hit with a slug, the kevlar vest might stop it. But you might have a big bruise underneath. The area of the blast also gets pretty hot and this can damage the paint/primer as well.
------------------ Wayne Webb Webb Sign Studio,Inc. creators of "woodesigns" "autograph your work with excellence" webbsignstudio@digitalexp.com
Posts: 7404 | From: Chipley,Florida,United States | Registered: Oct 1999
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I spoke to the customer service folks at Precision Board, who suggested that the primer/filler needs to be dried with a fan or heat lamp immediately before applying the stencil. According to them, the heat of blasting causes moisture in the primer to expand and break the adhesion to the HDU. They seem to think that the primer will absorb atmospheric humidity, even after drying, if not "force dried" just before masking.
I've used Jay Cookes primer and had the same trouble; much prefer the Coastal FSC-360.
Thanks for the suggestions.
------------------ "A wise man concerns himself with the truth, not with what people believe." - Aristotle
When the going gets weird, the weird turn pro. - Raoul Duke (Hunter S. Thompson)
Cam Finest Kind Signs 256 S. Broad St. Pawcatuck, Ct. 06379 "Award winning Signs since 1988"
Posts: 3051 | From: Pawcatuck,Connecticut USA | Registered: Nov 1998
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I've had the same problem. Too high a blasting pressure will definatly cause paint peel up.
I've noticed that after sanding the HDU, no matter how much vacuuming and blowing with the air gun, there is still dust on the panel. Static electricity is the culprit. One day I prepped a panel and after sanding, vacuuming and hitting it hard with the air gun, I took it outside and used the garden hose nozzle with a hard spray. A river of HDU dust came off of the panel. Stood it up on the sunshine and it was dry and ready to paint in less than an hour. So maybe your panel is not dust free for the paint to stick well in the first place.
Hmmmm... what am I going to do this winter?
------------------ Dave Sherby "Sandman" SherWood Sign & Graphic Design Crystal Falls, MI 49920 906-875-6201 ICQ: 21604027 sherwood@up.net
Posts: 5397 | From: Crystal Falls, MI USA | Registered: Apr 1999
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