If you can provide some photos, it would let us see what you are seeing.

 

To me, sorching/burning occurs when the blade is not cutting fast enough. This can be caused by a dull or dirty bit, excessive speed, force or depth of cut for the hardness of the material.

I clean my blades as I know oils/resin from prior cuts easily build up and can create a trailing edge that overheats and changes the exit profile of the blade possibly depositing burned resin on the work.

Don't get me wrong, my novice work is seldom of the finest quality - but I do know the disappointment of seeing a burn mark that I just can't sand out.

Our hobbies are not a race - our tools are the critical aspect of the work. I'd suggest inspecting and cleaning, slowing down the process, lightening the pressure/depth of cut and replacing questionable bits/blades before they give your new work more problems than you and it deserve.

For some reason, I'm reminded of a video I had seen years ago of an old world-style wood worker that had completed a good pass with his plane. The shaving was even in width and curled admirably, the sound of the blade against the wood was a constant "sssshhhhh" and when the stroke was completed the fellow took the time to feel the finished cut with a free hand rather than take a second cut immediately. That kinda sticks in my head. Working with the material, not against it.

 

Photos

 

Thanks - can you show the whole project?

 

 

Looks like burn marks, crummy bits or wrong speed either motor or how fast you feed.

 

Why don't they show up before I apply the finish? When sanded to 180, those areas looked like the unburned areas.

 

What is the bottom wood? In addition to the wood burning, you may be experiencing sawdust from the bottom being forced into the grain of the maple and the bottom wood dust darkening with the final finish. Ways to prevent this would be, using a tack cloth to remove any dust and only wiping with the grain, and making sure the router is only cutting with the grain to avoid the same thing as the tack cloth.

 

The bottom wood is Padauk. I was careful to remove all the red dust from the project before finishing. I did use a vacuum and a tack cloth. There was no dust on the project when I applied the finish.

 

Are you experiencing this phenomenon on other projects as well?? Or, just this one piece.
Great looking project, BTW.

 

I experienced it on some other projects that used Maple. I think pt is probably from a slow feed rate around the curves.

 

I've never experienced a burn showing up after sanding. It seems plausible that it could be a chemical reaction between the finish and the burned fibers. In your photos, it doesn't seem to matter whether it is end grain or not. When you are sanding those areas, are you sanding by hand? Or are you using a spindle sander? Or..

 

Is using power tools for the final shape a must? What if you used power tools to get the bulk of the material out, and then wood carving chisels and/or sanding blocks to get the final shape?

 

Some of the burn marks look like areas where the grain direction isn’t in your favor. Maybe try to flush out the problem areas before you invest a lot of time in the finish process. If you can reveal them use a cabinet scraper take out the burnt fibers. Sanding just grinds the burnt dust deeper.

 

I'm surprised that you don't see those with a mineral spirits wipe, I would have thought they would should show right up.

I understand your problem though, hard to remove what you cannot see!

You could always do a thin coat of shellac that you then sand off. It dries quick. Might show up the problem areas where you need a bit more work.

Ideally, better/sharper/cleaner bits as everyone is suggesting, but some burning is inevitable (maybe I need sharper and cleaner bits too!).

For removal of the marks, that looks like just the job for a rasp. I like hand cut ones the best, the slight randomness of the teeth makes them cut smoother. They're expensive though, that hurts. Lee Valley has the cheapest versions, made in the Czech Republic. Rasp the marks out, re-checking with something that makes them show up, then sand down, starting maybe a bit coarser than you might otherwise to get the rasp marks out. A card scraper will also get the rasp marks out.

If the rasp is too spendy for now, try to find a coarse half round bastard file. Those work well if they are aggressive enough. Or a cabinet scraper. Most come with a ready made hook these days so one of them will get you out of the woods.

Or there's AzureSkydiver's suggestion, another viable option if you have some sharp chisels. All different ways of skinning the same cat, so long as you know where to find that cat. Getting the marks to show up before you're done is the key thing there.

 

I make these same type bowls. I generally get the burn marks when I and either going too slow with my feed rate, going against the grain, or using a dull bit. I tend to get the burn marks on the bowl bottoms quite a bit. I will shape the bowl down to the last pass or two with the used bit, then swap it out for a new sharp bit to make the last 1/64" finishing pass, moving quickly and being careful in the corners, same with the round overs on top, first pass removing most material and second pass cleaning it up and moving fast. Starting this years batch

 

It might be moisture hiding in pockets of denser wood. The bit heats it, it cooks, but can't be seen until later. If you're up to drying your wood before assembly, if your wife will let you, put it in the oven and cook it at a low temp for a few hours. You can't lose anything if it doesn't work. Just a suggestion. ............ Jerry (in Tucson)

 

Everybody above has offered the same suggestions that I would have made myself. I have problems with handheld routers burning maple, cherry, and sometimes walnut.

What works for me best is to use a variable speed router and slow the router motor speed down.

The parts that tend to burn are where I am moving the router and follow curves or go around corner, etc. It is so natural to slow down how fast you are moving the router as you negotiate curves, corners, etc. ... and then the wood burns. Cutting motor speed helps for me.

I also wanted to mention your router base. Having a larger base gives you confidence against tipping as you move the router around tight curves or corners, etc. Having the confidence to keep a steady, constant router motion even in tough tight spots may help reduce burning, too.

 

I've had troubles with maple too. The burn marks can be deep. I don't know if it's because of sugars in the wood or just the nature of maple, it can be aggravating. What I do is try to keep everything cool, daddy-o. I sand slower if using my drill press as a spindle sander, I make sure my router bits are sharp. I try to keep a good feed rate, but that is challenging when the shape is curved and changing grain direction a lot. Seems like burn marks happen around corners where I'm taking care to not get tear-out.

I'm not sure why they show up after applying finish, and don't show up when wiping with mineral spirits. I don't think any strange chemistry is happening so it must be that the finish is letting the light through. Applying a finish will increase contrast and that is what I think is happening.

Does Zinser seal coat let the marks show through? It's a shellac product but has very little color. You might try putting that on the maple to show the marks before you final sand, and sand it all off before finishing.

 

I've had troubles with maple too. The burn marks can be deep. I don't know if it's because of sugars in the wood or just the nature of maple, it can be aggravating. What I do is try to keep everything cool, daddy-o. I sand slower if using my drill press as a spindle sander, I make sure my router bits are sharp. I try to keep a good feed rate, but that is challenging when the shape is curved and changing grain direction a lot. Seems like burn marks happen around corners where I'm taking care to not get tear-out.

I'm not sure why they show up after applying finish, and don't show up when wiping with mineral spirits. I don't think any strange chemistry is happening so it must be that the finish is letting the light through. Applying a finish will increase contrast and that is what I think is happening.

Does Zinser seal coat let the marks show through? It's a shellac product but has very little color. You might try putting that on the maple to show the marks before you final sand, and sand it all off before finishing.

I will try the seal coat. Perhaps that will help. Thanks

 

Curved scrapers can sometimes do wonders. Explicatives also help.

 

wichman3 has a possible good explanation. When I get those marks, it's when I pass too slowly, causing burn marks. But, I'm not routing a light colored with a dark colored wood.

I sometimes wipe a light coat of Rubbing Alcohol or Denatured Alcohol on the wood to see what it would look like. Before I apply the Natural Stain.

 

At a first glance I noticed parallel lines on the top surface. My first thought is sticker stain.

 

I agree with Aj2. It looks like changing grain direction may be rearing its ugly head. I don’t know if your method of work would permit it but my thought would be to try using compression bits in the router. These things do a really good job. Their offcut is not sawdust but shavings because they shear the wood rather than cutting it, like a helical head jointer or planer. Another possibility would be to try soft maple. Not all of it has the curly figure but even if it has some, if it doesn’t burn, you might get a bonus in your project from the added figure. Good luck.