If the card is your property you can do whatever you want with it.
HOWEVER
If you go onto sell, trade, or giveaway said card it is absolutely imperative to disclose what has been done to the card.
The reason being is that the condition of a card results in a higher monetary value.
The better the condition the higher the value.
If someone is altering a cards condition through a repair or restoration process, its actually not restoring anything.
What its more accurately doing is masking, concealing, or hiding the damage from detection.
If you were to examine the card via x-rays and other methods, you would be able to determine there is infact structural damage to the integrity of the card board and its just been low heat soldering penned / ironed out.
So if someone is masking, concealing, or otherwise hiding the true condition of a card for a higher monetary gain. This is a form of deciet, which is a lie, and because there is money involved its also fraudulent.
The same could be said for surface gels, ointment, and creams. The micro surface flaws are still there, they are just being masked with a filler, which is a substance not original to the card.
By most grading standards this is considered an alteration. Thus the card would be designated “Authentic but Altered” basically making its value zero.
The issue is the grading companies dont have a 100% fool proof process of finding these, and people exploit that, despite it being in the terms of service.
If you aren’t disclosing the alterations to the card, people are likely going to buy it on a false pretense that its entirely all original and that no changes or additions have been made, wether you call those alterations “restoring”, “repairing”, or anything else with a more pleasant ring to it. The end result is someone paying more money on a false pretense because they were decieved into believing the card was entirely all original. (Again not disclosing the changes, is deciet, which is a form of lying)
Keep in mind even collector cars MUST disclose if its all original or a restoration parts. (People often make this comparison to car restorations, but leave out how it still impacts values and collector interest).