Finish heat resistance?

[decibel]

I've replaced the top from the my first kitchen table I made a year go. Wasn't the best idea for that to be my first project ever but hey I learned plenty. Now I made the new top out of black walnut and put on my finish. Here's the finish schedule I used.

2 coats of garnet shellac ~1.5# cut (spray)
2 coats of blonde shellac with some dark walnut transtint (sprayed)
4 coats of target finishes waterbase glossy poly
1 coat of waterbase satin poly.


I was using oil based poly before (arm-r-seal) but couldn't but have wiping smudges everywhere when I used the satin for the last coat and I stupidly sat it outside to try to spray since I was having no luck and the sun took care of "stripping" off the poly.

Anyway here's my real problem. I'm afraid the poly doesn't have enough heat resistance as I noticed one spot where some hot liquid must have hit it and burned through to the glossy poly underneath. I also noticed today the finish would turn cloudy white when warm plate was on it (like pancakes etc) but discoloration went away as it cooled back off but I'm worried that's just a warning sign. Soooooo would an oil base poly have been better? I was going to use Rock Hard Varnish or maybe even waterlox but wanted to see how this poly would hold up since I have some and haven't used it.

If I decide to go with one of the short oil varnishes would I need to strip completely? Seemed like they wanted bare wood and I liked the color with the shellac so I just went poly :dontknow:. Soo I take it wouldn't be advisable to use Waterlox or Rock hard with shellac underneath?

 

[pslamp32]

I've used just about every finish over shellac with no adhesion issues as far as I remember. The question I have is how you plan to get the poly off without doing any damage to the shellac undercoat...also, with so many coats of poly, did you allow proper drying time between coats? Sounds like that may be part of the problem...

 

[decibel]

I've used just about every finish over shellac with no adhesion issues as far as I remember. The question I have is how you plan to get the poly off without doing any damage to the shellac undercoat...also, with so many coats of poly, did you allow proper drying time between coats? Sounds like that may be part of the problem...

Doesn't seem to be an adhesion issue at least I don't think so since it's not flaking off or anything of that nature. IF I do end up stripping it (which will be quite a bit down the road) I wold take it down to bare wood. Just wasn't sure if shellac under one of the short oil varnishes would cause problems or make it less resistant to heat. It doesn't appear to be any damage to any of the poly coats underneath. The coats of poly are pretty thin since I sprayed them and didn't want a plastic looking finish but the top satin coat is only 1 thin layer. Maybe I should have put on a few more coats of the satin but since poly doesn't burn in I only used that topcoat to get the sheen I wanted...
I'll get some pics up here.

 

[walnutjerry]

I think the heat from whatever produces a amall amount of moisture (sweat) ----that does not go well with shellac. As a result you get the "white spots" usually the size of whatever sat on the unprotected finish, as in the diameter of a coffee cup or the bottom of a wet glass.

Just my opinion from seeing the white spots on other table tops.

Jerry

 

[pslamp32]

Doesn't seem to be an adhesion issue at least I don't think so since it's not flaking off or anything of that nature.

Was referring to your question of using a varnish over shellac.

 

[pslamp32]

I think the heat from whatever produces a amall amount of moisture (sweat) ----that does not go well with shellac.
Jerry

In the OP's case the shellac appears to be covered by quite a few coats of poly so it shouldn't matter IMO...

 

[pslamp32]

Also, the reason I mentioned drying time was because I've noticed weird anomolies with my finishes in the past when I didn't allow proper drying times. I believe I've read that it should have some time to actually cure between coats not just 'dry to the touch'. Topcoats can suppress full curing of previous coats if I remember correctly...poly used to give me all kinds of problems which is one reason I never use it anymore...

 

[kooshball]

I think the heat from whatever produces a amall amount of moisture (sweat) ----that does not go well with shellac. As a result you get the "white spots" usually the size of whatever sat on the unprotected finish, as in the diameter of a coffee cup or the bottom of a wet glass.

Just my opinion from seeing the white spots on other table tops.

Jerry

I am thinking that this might be part of the root cause as well; did you notice any condensation in the impacted areas? I am not sure about the shellac but the softening point of most polyurethanes is going to be above that of boiling water and even if it softens that doesn't mean that it will cloud. What is the exact brand name / type of top-coat that you used?

 

[Gofor]

I am thinking that this might be part of the root cause as well; did you notice any condensation in the impacted areas? I am not sure about the shellac but the softening point of most polyurethanes is going to be above that of boiling water and even if it softens that doesn't mean that it will cloud. What is the exact brand name / type of top-coat that you used?

Realize that water-based "poly" is really an acrylic plastic coating, which has significantly different properties than the oil-based "poly", which is actually an organic varnish, (don't know its heat resistant mar point), and 2 or 3 component catalyzed isocyanate "poly" which has a heat resistance to over 200 degrees.

IIRC, the oil based poly has greater moisture resistance than the water-based acrylics.

Go

 

[decibel]

Thanks for all the ideas. Futher updates is that it seems to be more of the water than heat as I get cup rings that will hang around for may 30 - 45 minutes after the cup is removed and then they fade away. The waterbased poly I'm using the the EMTECH 9000 from target coatings.

here's the pics pretty bad quality...




here's one of the inital damage not sure where it came from though..




So I"m still not sure if I chalk it up to waterbase poly or how I applied it. Before putting it on the last satin coat it had a film like feel to it but after the one satin coat it almost feels very flat and not "plasticy". Is there any reason to put more coats of satin on I'm not sure I understand why we do multiple coats of poly since it doesn't "burn in" like shellac or lacquer?:dontknow:

 

[pslamp32]

Why not use the waterbourne 'lacquer' they sell? Since you're spraying I don't see the advantage of using poly. I use the the 'lacquer' all the time without any real issues. So much easier to repair than poly...

 

[Howard Acheson]

Two points. First, there are few finishes that will withstand heat much above 150-170 degrees. That's the reason for using trivets, place-mats, coasters or some other insulating device. Cold, wet items like iced drinks will also damage many surfaces.

Second, other than true lacquer or dewaxed shellac, film fiinishs continue to cure and don't develop their full protective qualities for 3-4 weeks. Finishes are particularly sensitive to heat and moisture until they are fully cured. A full cure is needed with any waterborne finish and even then, it is not generally as water and heat resistant as an oil based finish. Shellac containing its natural wax is no where near as water and moisture resistant as a dewaxed shellac.