There's always something nerve racking about art projects that involve the use of permanent ink on expensive paper or card. If something goes wrong, it's not like you can rub off ink from paper or cardboard like you can with pencil marks. Although that's true enough, several techniques can help eliminate ink stains and spills -- although they're a little more complex than simply using an eraser.
Spirit of Salts
Spirit of salts is an old-fashioned name for hydrochloric acid. Dissolve 1 part of hydrochloric acid in 4 parts of water and use a cotton swab to gently remove ink from the paper. Be careful to only apply this solution to the ink you want removed, and it's best to test on a sample piece of the same paper beforehand to make sure it won't damage or mark it further.
Traditionally, the easiest way to remove ink from paper or cardboard was with a metal scraper, or sharp ink eraser, which removed the top layer of paper or cardboard, including any ink that was on it. This method requires a delicate hand and weakens the paper at the area of removal, but it has the advantage of ridding any type of ink.
Chemical Ink Eradicator
Chemical ink eradicators can be purchased from most stationery stores, and use chemicals to alter the bonds of royal blue fountain pen ink -- rendering it invisible. This has the advantage of leaving preprinted markings largely unaltered; although other forms of ink, such as from a ballpoint pen, will turn brown if the chemical ink eradicator is applied to them.
Most types of ink can be removed from paper or cardboard with a bleach pen, like those used to apply to stains "on the go." Check for a brand that has the finest tip available. This will remove or fade ink on the first or subsequent applications. Be sure to test it on a corner of the paper first, however, to make sure the bleach doesn't discolour the paper itself.
Alcohol or Solvent
Almost all ink is made from an alcohol base, which evaporates quickly, leaving the ink mark in place. Applying alcohol or solvent (such as lighter fluid) on the ink stain will reactivate the ink molecules and you may have some success dabbing the stains off with a paper towel.