Black marks while sanding shellac

[Ron Swanson Jr.]

I finished my last project with shellac, and some black marks surfaced while sanding that have me scratching my head. 

I am definitely not an accomplished finisher, so I'm hoping someone can shed some light. 

Here's the facts: 

1. I applied 5 coats of thinned shellac to Cherry. Coats 1 &2 were thinned 1:3 (1 part shellac, 3 part DNA). Coats 2&3 were 1:1 and final coat was back to 1:3. 

2. I used 1000, 1500 and 2000 grit sandpaper on a flat block for VERY light sanding in between coats, mostly to knock down inconsistencies and high spots in the application. 

In doing so, i got some black marks like this: 

These appeared almost exclusively on side grain and chamfers. I don't think i saw any on the face grain. 

They look like a light sanding would remove them, but they were really stubborn and difficult to remove, had to sand back to bare wood in some instances. 

I thought at first this was due to build up in the paper, but i got the same thing when using fresh sheets. 

Any idea what caused this and how to avoid in the future? 

Thanks in advance! 

 

[gee-dub]

Just like tooth count on a saw blade, more 'items' equals greater heat.  Finer grits generate more heat per stroke.  Light sanding with a block shouldn't really create this problem regardless of the grit.  Shellac has the wonderful property of "drying" very quickly and allowing us to move forward.  However, a full cure can take 24 hours or more.  The marks seem to cross grain patterns so it looks like an influenced mark rather than a "burned" set of sugars in the wood. 

I don't think any of these observations are helping you and I apologize for that.  I experience small burns or burn patterns during fine grit sanding.  Generally moving up a grit will clear them.  There are times when, like you, I have to use a card scraper to get back through the layers to the marked area and remove it.  Cherry is certainly burn-prone as is hard maple.  Marks appearing after multiple coats of finish is odd and I will be following this to see what you find out.

[Ron Swanson Jr.]

@gee-dub

That IS helpful, as it gets my mind wandering down hallways i might not have gotten to on my own. 

And yes you are on the money, i definitely did not wait 24h in between coats for a full cure, and that's something that hasn't occurred to me. 

I agree with you about cherry, it has been my experience that all the fruit woods I've worked with have been pretty burn prone. 

Thanks Glenn, i appreciate your advice and experience as always!

 

--Doug

[gee-dub]

On 6/23/2025 at 7:28 AM, Ron Swanson Jr. said:

 i definitely did not wait 24h in between coats for a full cure, and that's something that hasn't occurred to me. 

 

Don't get me wrong; I use shellac's rapid dry-to-touch time to my advantage frequently.  I just don't apply a different finish on top till the next day.

[Ron Swanson Jr.]

On 6/23/2025 at 9:57 AM, gee-dub said:

Don't get me wrong; I use shellac's rapid dry-to-touch time to my advantage frequently.  I just don't apply a different finish on top till the next day.

I think you may have hit the nail on the head there. 

If i remember, those marks appeared after sanding my thicker coats. They were dry to the touch but sanded after about 1h drying. 

i can try a ittle experiment to confirm, but i suspect that's going to end up being the culprit.