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Sanding sealer


ADFinlayson
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You are likely to get a lot of various suggestions on the grit. Many say 220 is high enough, and I like highly polishing the wood to get the most chatoyance before any coating goes on. And most will say somewhere in between. At least sand it enough that you can no longer see sanding scratches with the naked eye. And clears will magnify sanding scratches.

SR

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  • 3 months later...

Don't post often but I felt the need to warn you:

Don't use a sanding sealer under polyurethane, the poly won't bond well and you are almost definitely going to have adhesion issues. Your top coats will likely start peeling in a matter of months, or sooner. You actually don;t  need a sanding sealer, it has no function for what youre trying to do. If you feel the need to wash coat with something to lock in oils of the wood or something,  you can do a washcoat of shellac and sand it flat, shellac will has no issues with a poly coat on top. Most likely its fine to go straight to the poly on bare wood, your first coat you can consider your sealer coat, sand it flat, and keep on building film with additional applications,  with a bit of sanding in between coats to remove dust nibs, high spot, generally keep it level before a final sanding and buffing once all the coats are finished.

 

As for sanding, it makes no difference how high a grit you sand to, as long as there are no scratches visible. If you go to way too high of a grit, like to the point where youre polishing the wood, you may have adhesion issues with the finish. Once the film finish is on, there is no difference between wood sanded to 150, 180, 200, 240, whatever, so long as there are no scratches seen.

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3 hours ago, FreshyB said:

Don't post often but I felt the need to warn you:

Don't use a sanding sealer under polyurethane, the poly won't bond well and you are almost definitely going to have adhesion issues. Your top coats will likely start peeling in a matter of months, or sooner. You actually don;t  need a sanding sealer, it has no function for what youre trying to do. If you feel the need to wash coat with something to lock in oils of the wood or something,  you can do a washcoat of shellac and sand it flat, shellac will has no issues with a poly coat on top. Most likely its fine to go straight to the poly on bare wood, your first coat you can consider your sealer coat, sand it flat, and keep on building film with additional applications,  with a bit of sanding in between coats to remove dust nibs, high spot, generally keep it level before a final sanding and buffing once all the coats are finished.

 

As for sanding, it makes no difference how high a grit you sand to, as long as there are no scratches visible. If you go to way too high of a grit, like to the point where youre polishing the wood, you may have adhesion issues with the finish. Once the film finish is on, there is no difference between wood sanded to 150, 180, 200, 240, whatever, so long as there are no scratches seen.

Interesting, would it not depend on what the sealer base chemical is to determine adhesion? Ie an oil based poly would not adhere to a water based sealer, in the same way a polycrylic wouldn’t adhere to an oil based sealer? 

I will monitor this as I’ve just finish as guitar in this way 

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Unfortunately, its the stearates in the sealer which are added to ease sanding that makes the  poly have a hard time bonding. Sanding sealers you buy in a can are usually just a varnish with stearates added, which prevent the sanded finish  from sticking to itself and gumming up. For 99% of amateur woodworkers there is almost never a need to use a dedicated sanding sealer product, any finish can be used as a seal coat or barrier coat in the right contexts.

I've seen poly over sanding sealer before, you could literally pick the finish with your finger nail and peel off massive flakes at a time. Having said that, I hope I'm wrong because its no fun doing things twice, I guess just keep an eye on it, maybe the specific products youre using dont fall under what Ive described. 

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