Author Topic: Surface prep for plywood  (Read 1454 times)

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Offline mrFinpgh

  • Posts: 216
Surface prep for plywood
« on: June 10, 2017, 10:38 AM »
I'm going to be finishing some walnut plywood with Osmo Polyx in the near future.   Most of the panels are about 3' x 1', but going as large as 2' x 5'.

I have a RO150 and the Pro5.   I usually prefer the RO150 in rotex mode when I'm preparing to finish solid wood, but haven't tried using it for plywood yet.  The Pro5 is a decent sander, but the RO150 always feels more stable for me.

My concern is that even beginning at 180, the RO150 could potentially go through the veneer.   I'm consistent about keeping the sander moving, but don't know if this is going to be an issue.  How much risk is there to eating through the finish with 180g in Rotex mode?   Given the finish, is there much upside to going up to 220?

Thanks,
Adam

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Offline Tim Raleigh

  • Posts: 3452
    • Oakville Cabinetry
Re: Surface prep for plywood
« Reply #1 on: June 10, 2017, 11:11 AM »
How much risk is there to eating through the finish with 180g in Rotex mode?   Given the finish, is there much upside to going up to 220?

There is not a lot of risk of going through the veneer if you keep it moving. I would think that starting at 180 in Rotex mode you will see micro swirls, so I would finish your sanding sequence in the orbital mode. Keep your hook and loop sanding pad clean. I am assuming this is a commercially produced cabinet grade veneer.
Tim

Offline mrFinpgh

  • Posts: 216
Re: Surface prep for plywood
« Reply #2 on: June 10, 2017, 11:50 AM »
Thanks, Tim.

It's this product: http://www.columbiaforestproducts.com/product/purebond-classic-core/

Seems like the veneer is on par with the other cabinet grade plywood I've gotten from better lumber dealers. 

I'm curious about this statement:

Quote
I would think that starting at 180 in Rotex mode you will see micro swirls, so I would finish your sanding sequence in the orbital mode.

On a couple test pieces that I did, I noticed that at some angles, there were a visible circular pattern.  It's slight, but noticeable.  I assumed this was because the sample pieces I prepped were not properly restrained while sanding and resulted from vibration.  It sounds like maybe there's more to it than that?   

I usually avoid Random Orbit mode, as it seems like I usually get less consistent results.  But I also usually work through a fair number of grits in the process when using Rotex Mode.   You've got my curiosity piqued - what would be the reason for micro swirls when starting at 180 in Rotex?  Is it because the plywood should be prepped at lower grit first?

I'll be experimenting on some more cutoffs and cabinet backs on the specific product, but this definitely helps me save a little time!

Thanks,
Adam





Offline ear3

  • Posts: 3210
Re: Surface prep for plywood
« Reply #3 on: June 10, 2017, 12:49 PM »
I generally start at 150 with veneered ply.  Before starting in on Rotex mode, I would just double check the face veneer thickness by eyesight.  A visual inspection will tell you immediately whether you have anything to worry about as far as burning through the surface. 

I use the ETS-EC more for finish sanding now, but when I was working primarily with the RO150, I usually made my last pass in RO mode rather than geared mode.  I know some people on the FOG say they don't get swirls even in geared mode, but that's just not my experience.
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Offline Holmz

  • Posts: 3749
Re: Surface prep for plywood
« Reply #4 on: June 10, 2017, 08:36 PM »
The Festool or Mirka hand blocks that hook to a vacuum are not fast, but you can go with the grain as tenderly as you like.
Of course I used a random-orbital the other day with 240 on jarrah veneered ply, and did not consider using the hand block ... But I probably should have. ;)

Offline Tim Raleigh

  • Posts: 3452
    • Oakville Cabinetry
Re: Surface prep for plywood
« Reply #5 on: June 11, 2017, 01:29 PM »
You've got my curiosity piqued - what would be the reason for micro swirls when starting at 180 in Rotex?  Is it because the plywood should be prepped at lower grit first?

In my experience using the Rotex alone seems to create micro swirls. This is more evident in clear coated hard maple than in say walnut but it is still there. I have found that sanding (cross grain pass and then with the grain pass with overlaps) in Rotex mode and then switching to Random orbit with the same grit to reduce scratch marks and tails more than any other method. Making sure the pad (Rotex and ETS) you stick the paper to is clean and lint,dust, chip free etc, also insures that the pad is even on the sander creating little to no swirls or tails.
When using water bourne coatings I generally water pop the veneer which raises the grain,  I sand to 120 then apply a sanding sealer and sand that at 320-400.
Tim
Tim

Offline Joseph C

  • Posts: 260
    • Integrity Design+Build
Re: Surface prep for plywood
« Reply #6 on: June 11, 2017, 01:50 PM »

I usually avoid Random Orbit mode, as it seems like I usually get less consistent results. 


You AVOID Random Orbit mode???
That's so strange - if all you need is rotary mode you might as well be using a belt sander ;)

With the plywood you're talking about, you don't need rotary mode at all, and you can start at 150g.  That material is so smooth you could also just sand with the grain by hand.
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Offline Holmz

  • Posts: 3749
Re: Surface prep for plywood
« Reply #7 on: June 12, 2017, 04:08 AM »
...
You AVOID Random Orbit mode???
That's so strange - if all you need is rotary mode you might as well be using a belt sander ;)
...

A belt sander seems like the wrong machine, but the BS105 with the sanding frame can take belts in the 240, 320, 400 and 600 range.
There are probably worse tools to use than a belt sander, and the scratches all go in the direction of the belt.

In fact a belt sander (with a frame) may be the optimum/ideal tool for this?


Then there is also a 1/2 sheet sander (aka the R2S), which natural makes for flatter sanding, rather than just smoothing the wood and possibly creating ripples.
« Last Edit: June 12, 2017, 04:16 AM by Holmz »