Mirocat finishing query

Talk about musical instrument construction, setup and repair.

Moderators: kiwigeo, Jeremy D

Post Reply
vandenboom
Blackwood
Posts: 414
Joined: Sun Oct 26, 2008 2:27 pm
Location: Melbourne (Ringwood), Australia

Mirocat finishing query

Post by vandenboom » Sat Nov 11, 2017 12:53 pm

Strange thing happening in final sanding/levelling of a Mirocat finish today.
Sanding wet with P600 wet/dry. The paper clogs up pretty quickly and doesn't readily wash off when I dip back in water. Like lots of small pellets of finish.
sp1.jpg
Preparation to this point if it helps....
- new tin of Mirocat 60/40, 1:1 mix with 5% retarder added
- 5 coats, 1 week to dry, level with dry P320
- 5 coats, 2 weeks to dry
It is levelling but I'm going through a ton of sandpaper. Using small sanding block with rubber base.
Any thoughts?
thnx. Frank.

simso
Blackwood
Posts: 1768
Joined: Mon Apr 25, 2011 10:36 pm
Location: Perth WA

Re: Mirocat finishing query

Post by simso » Sat Nov 11, 2017 1:42 pm

Finish is still too soft, why its soft cannot answer that one, also are you adding some detergent into your water for sanding, to help break the tension

Steve
Steve
Master of nothing,

Do your own repairs - http://www.mirwa.com.au/How_to_Series.html

User avatar
Trevor Gore
Blackwood
Posts: 1605
Joined: Mon Jun 20, 2011 8:11 pm

Re: Mirocat finishing query

Post by Trevor Gore » Sat Nov 11, 2017 1:47 pm

Finish too soft
Pressing too hard
"Scrubbing" rather than sanding (covering the same patch too often, without clearing the paper)
Sanding block too hard
Not wet enough
No wetting agent (soap) in the water
Crappy sand paper

Any or all of the above. Then ask yourself the next "Why". (e.g. Why is the finish too soft?)

Just to put it in perspective, it's possible to sand new shellac after about an hour (or even less) using good technique.

Just seen that Steve got there first :lol:

vandenboom
Blackwood
Posts: 414
Joined: Sun Oct 26, 2008 2:27 pm
Location: Melbourne (Ringwood), Australia

Re: Mirocat finishing query

Post by vandenboom » Sat Nov 11, 2017 1:49 pm

Thnx steve. Yep - drop of detergent added. Also soaked sandpaper overnight.
I assumed thinner & retarder doesn't have shelf life - over 3 yr old.

simso
Blackwood
Posts: 1768
Joined: Mon Apr 25, 2011 10:36 pm
Location: Perth WA

Re: Mirocat finishing query

Post by simso » Sat Nov 11, 2017 2:19 pm

Trevor Gore wrote:
Sat Nov 11, 2017 1:47 pm
Just to put it in perspective, it's possible to sand new shellac after about an hour (or even less) using good technique.
Spot on, even nitro can be sanded a few hrs after painting with the right technique

Steve
Steve
Master of nothing,

Do your own repairs - http://www.mirwa.com.au/How_to_Series.html

vandenboom
Blackwood
Posts: 414
Joined: Sun Oct 26, 2008 2:27 pm
Location: Melbourne (Ringwood), Australia

Re: Mirocat finishing query

Post by vandenboom » Sat Nov 11, 2017 6:44 pm

Thnx Trevor for all the possibilities.
The frustrating thing about 'teach yourself finishing " using the written words of others is that i can never be too sure about too soft/too hard, too wet/too dry etc.
However I haven't experienced this before on about 12 guitars - technique & sanding block are the same.
Sandpaper - flexovit purchased from autopaint supplier - so hopefully not crappy.
2 week cure time considered adequate.

So I'm left wondering why the finish could be too soft after 2 weeks in melbourne oct-nov climate and fresh pot of mirocat .

I take yr other point though that, in discovering it is too soft, whatever the reason, a change of technique might enable one to push on! Eg. Perhaps less pressure plus lower grit. I guess i got there in the end by using lots more paper (than before without knowing what the norm is anyway)

God how i would love to be able to work thru all stages of a guitar finish under the watchful eye of an expert.

User avatar
Trevor Gore
Blackwood
Posts: 1605
Joined: Mon Jun 20, 2011 8:11 pm

Re: Mirocat finishing query

Post by Trevor Gore » Sun Nov 12, 2017 11:45 am

Frank, when I was building boats, I used rubber blocks most of the time when sanding, usually wet. For some reason, they never seemed to work particularly well for me on guitars. Probably too hard. I have a variety of cork blocks that I use mostly and when sanding through the grades finishing, especially on softer finishes, I use some plastic foam packaging material between the cork and the paper. That's the set up on the left of the pic below. This sort of arrangement rides more smoothly over the finish without causing the blobs on the paper that you have been suffering. Of course, a soft block like this won't flatten lumps, but those should have been got rid of way before, using a hard block. By changing the thickness of the foam packaging material, you can change the hardness to whatever suits the circumstances best. I can't remember what the packaging came with, but it's pretty common stuff, just an expanded plastic foam. Can be used wet (with water or other lubricants) or dry.
DSCF0266s.jpg
DSCF0266s.jpg (188.33 KiB) Viewed 9472 times
That cork block is flat on the bottom side and curved on the top, though that's hard to see. It's curved to fit guitar waists just by shaping it on a drum sander (pics in Chapter 4 of the Build book). The foam will wrap around the curved side, too, of course.

For very delicate surfaces I use the foam block on the right. It is very soft and about 45mm thick with a very flexible film abrasive stuck to it. Again, can be used wet or dry.

I've been sanding stuff for 50-odd years and I'm still learning!

vandenboom
Blackwood
Posts: 414
Joined: Sun Oct 26, 2008 2:27 pm
Location: Melbourne (Ringwood), Australia

Re: Mirocat finishing query

Post by vandenboom » Sun Nov 12, 2017 2:07 pm

Thnx Trevor.
I will try this approach to sanding blocks.
The main thing I have got out of all this, because of your comments yesterday, is that I need to adapt the approach better to what is happening and not be so formulaic/rigid.
Today I changed the technique - much wetter, lighter touch and more frequent dipping of paper in water. No clogging and was able to do a single grit on guitar and neck with 2-3 bits of sandpaper.
Thanks again for your help.
Frank

User avatar
Trevor Gore
Blackwood
Posts: 1605
Joined: Mon Jun 20, 2011 8:11 pm

Re: Mirocat finishing query

Post by Trevor Gore » Sun Nov 12, 2017 4:22 pm

Just to give a bit more perspective about the life you can get out of good sandpaper...

I keep my wet use paper in an old plastic ice cream tub, full of water. Most of the pieces I have in there, ranging in grit from 400 to 2000, will do 4-6 guitars before getting chucked. That was a major revelation a few years ago about not using paper wet until it had at least 24 hours soaking. Now I just store it all wet, ready for the next use. After about a year, the paper backing begins to break down.

Post Reply

Who is online

Users browsing this forum: Google and 91 guests