# Urgent Help Needed - Polyurethane on Wenge



## nugget (13 Nov 2011)

Hi

I need some HELP !! having a bad day so far....

I'm finishing some wenge with plasti-kote polyurethane spray cans (from B&Q)
So far - 
Sprayed one pretty heavy coat to act as a sealer to fill the open pores
Left one week to dry, temp has been around 10 deg can does say min 20deg
Sanded flat without getting back to wood but pores filled nicely, no chemicals used to clean off just dusted heavily
Sprayed again with same varnish but new can (temp today 16deg) with light but wet coat and straight away the finish reacted like a chemical reaction raising and cracking !! not all over just in small patches.

I've finished other woods including oily rosewood like this with very good results and no issues and have sprayed is similar or lower temps.

Please help - 
Is it the wenge? if that was the case surly the sealing coat would have reacted worse
Is the the lowe temp? I cant see why this would cause it to react so quickly and havent had this before
Chemical reaction - didnt wipe down with white spirits or cutting compunds etc, same make of varnish but different can, could it just be a bad can? I have sprayed a small offcut with the same new can and it looks ok!
Bad technique?

any suggestions, I'm going to have to wait for it to dry off then sand back to the wood and start over again, problem is I've only got 2 weeks till the customer is coming! 

Thanks


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## yellowbelly (13 Nov 2011)

Whats the reccomended re-coat time with plasti-kote?
2 light coats are better than 1 heavy coat.
In all honesty its sounds like an adhesion problem, the suface has nt adhered to the wood, it will still dry but the 2nd coat has delaminated the first coat in places like you say, if you can get your thumb nail under the finish you will be able to peel it off


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## Streepips (13 Nov 2011)

Why a "pretty heavy first coat". That goes against all received wisdom.. Light coats and build up..... Sounds like you got a dried first coat and the second coat just softened it again. Because it wasn`t cured. Just dry.
So the next coat just softened it
Also, is it imperative that you use a plastic coat finish? I mean you might as well use melamine Wenge. Why not oil or shellac and/ or waxes?


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## nugget (14 Nov 2011)

Recoat time is within an hour or after 24hrs, I left it one week to be sure in the colder weather
It looks more like a chemical problem - I've seen it before long time ago spraying car paint on plastic motorbike panels with no plastic primer, it instantly cracks and raises.

the heavy first coat was purely to fill the pores, let it dry then rub it right down, then build up light coats on a totally flat surface for the actual finish.

The finish needs to be tough and waterproof hence the poly finish - plus it's (normally) easy to get a glass like finish 

I'm, obviously, no expert on finishes so tend to stick with what I know....

Thanks


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## Streepips (14 Nov 2011)

Well all considered then, I can`t really see anywhere where you went wrong. You say the crackling was in patches? Perhaps some contamination? Damp? Fly spray? Try rubbing down just one or two of the affected areas back to flat and try one more light coat, you should know pretty quickly if its going to happen again.
If it does, what about flatting it back then applying a layer of shellach ( or two?) flatting that and using it as a blinding coat for fresh poly? Shellac is inert and will not chemically react to anything. Just needs warmth and air dryness to apply.


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## nugget (14 Nov 2011)

Thanks - I'll look into Shellac if redoing the patches isnt going to help....


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## Chrispy (14 Nov 2011)

I think the solvent in the second coat has reacted with the first coat, possibly because the first coat was so thick that it hadn't cured properly, the surface that had cured you sanded off in places leaving patches of partly cured poly, this would be outside the one hour re-coat window but not fully cured ie out side the 24 hours.
If so I would recommend cutting back to a nice flat surface and then leaving at least 24 hours to make sure the exposed surface has cured then re-coating as per normal.


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## nugget (14 Nov 2011)

Chrispy":3q712kgn said:


> I think the solvent in the second coat has reacted with the first coat, possibly because the first coat was so thick that it hadn't cured properly, the surface that had cured you sanded off in places leaving patches of partly cured poly, this would be outside the one hour re-coat window but not fully cured ie out side the 24 hours.
> If so I would recommend cutting back to a nice flat surface and then leaving at least 24 hours to make sure the exposed surface has cured then re-coating as per normal.




Makes sense - I rubbed down the bad patches on Sunday so I'll leave it till middle of the week till trying again. If all else fails I'll start reading up on Shellac


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## yellowbelly (15 Nov 2011)

nugget":28x90m41 said:


> Recoat time is within an hour or after 24hrs, I left it one week to be sure in the colder weather
> It looks more like a chemical problem - I've seen it before long time ago spraying car paint on plastic motorbike panels with no plastic primer, it instantly cracks and raises.
> 
> the heavy first coat was purely to fill the pores, let it dry then rub it right down, then build up light coats on a totally flat surface for the actual finish.
> ...


If "the finish needs to be tough and waterproof" shellacs not going to help


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## Streepips (15 Nov 2011)

No disrespect or doubting your knowledgw YellowBelly, but I think you mught be ubderestimating the properties of shellac somewhat. Bear in mind that traditionally the only thing used ( and trusted enough) for repairing broken grindstones is shellac. It can be tough stuff in the right application....and I think as a blinding and bonding coat it is suited well. The waterproof aspect does not matter if the poly is coating over it.


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## yellowbelly (16 Nov 2011)

Streepips":3gajhcup said:


> No disrespect or doubting your knowledgw YellowBelly, but I think you mught be ubderestimating the properties of shellac somewhat. Bear in mind that traditionally the only thing used ( and trusted enough) for repairing broken grindstones is shellac. It can be tough stuff in the right application....and I think as a blinding and bonding coat it is suited well. The waterproof aspect does not matter if the poly is coating over it.



You must be using a different kind of shellac to what Im used to then  
I certainly would not be tempted to repair a grindstone with anything if its broken get a new one
Shellac was good for sealing naptha stains and then build a finish with sanding sealer(if you try sanding sealer without the shellac thats a BIG no no) poly on top of shellac is not reccommended and will lift the shellac while its curing :wink: poly coat on coat in light coats is the way to do it =D> does it mention sealing with shellac in the manufacturers instructions?


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