when sanding buy hand Theres a birthmark looking spot that comes out!?

[Badboy23]

Plz help, its the first time it happened to me. I first sanded the rough cast aluminum with 180,220,320,400. Now at 400 girt i can see it coming better.But at a certain spot there is like a birth mark that is a darker looking color like it just came out from under the aluminum when i was sanding. Anybody got a clue what this would be and how to fix it. I keep on sanding it but it stays the same color not like the rest of the surrounded area help.Thank u i'll show u a pic ,but if anybody ran through this before can u tell me what i should do.

[CarWiz]

What's the part? It could be in the aluminum itself which there is no fix or it may be an inclusion that's trapping oil. You might try baking the part by raising the temperature in stages. 150 for 15 minutes, 300 for 15 then 450 for 15. See if the part weeps oil or a contaminate from the area. If it does, continue baking at 450 until it's gone.

[Badboy23]

This the part someone help even though i go to a lower grit it does not come out when i was sanding with 400 grit by hand it just came out for no reason i don't know where it came from i went back to 320 and resanded it.It look like it went away but later on when the part drys it comes back ,what could this be? Can somebody please help me out i need help fast this bike neeeds to go. Also its part of the frame by your lower legs on a sportbike were i had to take out the rough casting .this is the part.

[Badboy23]

can u see it's that small rounded circle that is a little darker on left side

[CarWiz]

What happens when you spray it with water? Does the water bead up on the spot?

[CarWiz]

Forgot to ask: What's directly behind that area?

[Badboy23]

u know about sportbikes? Its the part were you leg is you know part of the frame and when i put water on it its normal it slides nicely man i just can't figure it out Carwiz. Man i gotta thank u so much for helping me though thank u man ur the only one .

[Badboy23]

it does not bead up, nothing is behind that area that its just the back side of the frame thats it .

[CarWiz]

Don't know about sport bikes but I know a little about metal. I take it that the part can't be removed to heat? Can you heat it with a torch? Is there a source of oil, coolant or heat behind that area? Is it welded to the frame in that area?

[Badboy23]

i think i can
but will it damage the aluminum