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I applied three coats of Minwax Gunstock stain on a Maple headboard and dried and standed between coats. Everything was looking good, so I applied a coat of polyurethane and thats when things messed up. Certain areas started to look discolored and patchy (see attached picture).

Is there anyway I can fix this or do I have to stand everything and start again? Not sure what I did wrong. I am wondering if it was becuase of sanding after the final coat of stain?
 

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It's called blotching. It's a characteristic of finishing maple. When you stain maple you need to use a pre-stain conditioner of some type or another. I haven't used any yet but a lot of folks like Charles Neil blotch control. I've always mixed boiled linseed oil and mineral spirits 50/50. Anyway from where you are the only fix would strip the bed with a chemical remover and start over. What happened is there are spots on the wood that are harder than others and the harder spots don't take the stain very good. The conditioner is a sealer which the soft parts of the wood soak up much more than the hard spots so when you apply the stain the density of the surface is more uniform.

It's not real bad, you could take the bed to a professional finisher and they could mix a toner and spray the lighter areas darkening to to match the darker spots.
 

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I've stained and finished a myriad of projects with Maple, in solid wood, veneer and plywood, where I didn't need a conditioner. Blotching could be from by initial prep work.

From your post, it sounds like your problem is from sanding the stain, and using multiple applications of stain. I wouldn't suggest sanding the stain at all. Oil base stains should be a one time application, and wipe on, and then wipe off excess.






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The thing you did wrong as already mentioned is not following the directions on the label. Stain is not paint. You apply it, let it set for 15-20 minutes and then thoroughly wipe off any excess. You DO NOT sand after applying the stain.

At this point, you should remove the stain using a chemical paint remove containing methylene chloride. Follow the directions on the label.
 

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If it was bloching shouldn't it have shown before applying polyurethane? In my case the blotches started appearing as the polyurethane started drying.

Also I did use Prestain, but wiping stain on and off led to very uneven texture so I just let the stain sit over three coats and build.
 

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If it was bloching shouldn't it have shown before applying polyurethane? In my case the blotches started appearing as the polyurethane started drying.

Also I did use Prestain, but wiping stain on and off led to very uneven texture so I just let the stain sit over three coats and build.
With any oil stain you wipe it off almost immediately. If you let it sit and soak it defeats the purpose of the wood conditioner.

The stain was blotchy before you put the polyurethane on it. It just shows up a great deal more with the poly on it. On natural wood it's known as making the grain pop but in your case made the stain pop.

What will happen if you apply stain to the surface and let dry the polyurethane will adhere to the stain instead of the wood and within a year start flaking off. The wood should always be wiped clean. The only stain you can lay over the surface is a gel stain.

When you use a wood conditioner you have to follow the directions pretty close. Some have you put the stain on within 30 minutes on some and some you let dry first. Then conditioners are like anything else, some are going to work better than others. Your preperation before finishing can affect it also. If you don't have the wood sanded to 220 grit it will be more prone to blotch and the conditioner may not be able to compensate.
 
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