Mildew
Eliminate by dabbing with 10 volume hydrogen peroxide or with dilute Chlorine bleach (depending on the care code of the article of clothing).


Rust
Use a commercially available anti-rust product.
Otherwise, apply a few drops of oxalic acid diluted in water (also called sorrel salt, found in hardware stores). Make sure to thoroughly rinse the zone cleaned! Warning handling of dangerous acid.
Use lemon juice on synthetic fabrics.


Sludge
Trick:
Before removing the stain, soften the stain with a fat (for example butter) then gently remove the excess.

Treat with a solvent or turpentine essence. Do not let solvents act too long (risk of fading on certain colours)
On synthetic fabrics, you may dab the stain with a rag soaked in either water with lemon juice or with White Spirit. Don’t forget to rinse!


Tar
Remove as much as possible of the stain by scraping. Soften the rest of the stain with some glycerine then wash normally. If the stain does not disappear, try applying special stain removing solvents.


Wax
Remove as much as possible of the wax layer, by gently scraping both sides of the fabric with the blade of a knife.
Then, place the fabric between two layers of absorbent white paper. Iron with a warm iron changing the paper as it absorbs the melted wax. Wash the fabric normally. If needed, final treatment as fat stain.