Finishing and Sealing Mahogany with Shellac

[Jim Haggerty]

Hello All,

I have recently been finishing some of my Indonesian mahogany walking staffs with Zinsser clear spray shellac. On the latest staff something odd happened.

I typically use a combination of exotic woods along with the mahogany and the exotics always finish out like glass. I stained the mahogany portions with a typical oil based stain. Later I started spraying the entire staff. After about the third coat of shellac when I came back after a drying period I found that the mahogany was covered with tiny bubbles, and when I say bubbles I mean actual spherical hollow clear bubbles smaller than the head of a pin. Each bubble was sitting on a pore opening in the wood. I assume that means the wood was outgassing something and it also tells me that the shellac was not particularly sealing the pores.

This is something I have noted with mahogany. The shellac never seems to fill in the pores despite wet sanding between coats. Instead it seems to continually migrate away from the pores no matter how many coats I apply.

Does anyone know what is going on here with the bubbles? Once I sand them off and respray the finish comes out okay, but it is still weird. Is there a good way to fill the pores in mahogany so I can get a glassy finish without the typical dimples at the pores?

One other question ... can I spray lacquer over shellac or vice versa? Are they compatible?

Thanks.

[Joe Chritz]

Not sure on the bubbles. Maybe the oil stain wasn't fully cured and that was off gasing some. If you don't give it a full 8 hours or more to cure try waiting a bit longer.

Shellac (dewaxed) can go over or under any other finish. It truly is a universal barrier coat.

The pores are a function of the wood used. Unless you use a dense wood with closed grain (maple is one) then you will have pores to deal with. To fill the pores you need to grain fill them somehow. There are a lot of products to do this with and a search on grain filler will yield a lot of info. I think there is a way to fill them with wet sanding with finish but haven't done it.

You can even tint grain filler to get different looks.

Joe

[Howard Acheson]

That is a very typical indication that the oil based stain deep in the pores has not fully polymerized. As it continues to polymerize, the oil in the stain produces heat and off-gases. That's what ends up being a bubble.

The solution is to let the stain dry longer and keep wiping the surface until all bleedback has ceased.

You have probably run into some deeper pored mahogany than you have worked with before.

[Sam Yerardi]

I agree with Joe & Howard. You can also try suscessive washings with mineral spirits, each time with a clean rag. The idea being to absorb a little more oil each time.

There is one other extremely remote possibility. I don't believe this is your case and I even hesitate to mention it but I do think it worth considering for any of us that use exotics. I've read too many articles too many times that describe how sometimes what we buy and THINK is one type of exotic hardwood may be a less-sold wood from the same area. In the Brazillian rainforest, in a two acre spread, there is typically over 250 species of hardwood. We here in America only see a minute fraction of that and probably will never see examples of a lot of this wood. Sometimes, a log importer might either inadvertently (or intentionally) import in a log that he claims is one type of well-known hardwood but it actually something quite different. My point is to keep in mind that when you have a piece of exotic wood (or any for that matter) that continues to respond differently than what everyone expects it to, it may be because it just isn't the wood you think it is. Rosewoods, for example, not only have different oils in them, but have anti-oxidants as well that unless are dealt with accordingly, will prevent ANY finish from adhere successfully. Sorry for the long-winded speech :).

[Jim Haggerty]

That is a very typical indication that the oil based stain deep in the pores has not fully polymerized. As it continues to polymerize, the oil in the stain produces heat and off-gases. That's what ends up being a bubble.

The solution is to let the stain dry longer and keep wiping the surface until all bleedback has ceased.

You have probably run into some deeper pored mahogany than you have worked with before.
Thanks, that is probably what happened. I was rushed and didn't allow enough stain cure time. Once the bubbles had been sanded it recoated fine, but the grain never filled. I can't do too much sanding or I go right through the stain. I'll have to experiment and see if I can shellac and sand alternately until the pores are filled but I'm back to bare wood, and then stain.

The pores aren't deeper than encountered before. All the mahogany in question is of the same source. I was just too quick to shellac I think.

Thanks.

[Jim Haggerty]

There is one other extremely remote possibility. I don't believe this is your case and I even hesitate to mention it but I do think it worth considering for any of us that use exotics. I've read too many articles too many times that describe how sometimes what we buy and THINK is one type of exotic hardwood may be a less-sold wood from the same area.Most all the exotics I use are easily identified since I tend to use the more extreme varieties. The exotics always finish very well. It's the cheaper woods I have issues with. ;) Oh yeah, and the %$#&^% spalted maple that has soft spongy areas that never seal up. :confused:

[Steven Wilson]

Jim, you'll need to look into using pore fillers with mahogany. I like the oil based pore fillers (van dyke brown is nice). If you don't like the carrier stain then apply a sealer coat of dewaxed shellac first. Time to take some scrap mahogany and try out different finishing schedules to see what works for you.