DIY high capacity 200V plus capacitor

I did a little experiment with 25cm square bits of Al tape dipped in a salt solution - it gave me a capacitance of around 5-6 uF.

If I was to use 2 x roll of Al foil (30m x 30cm) to make an electrolytic capacitor with a salt solution soaked insulator between the sheets then I should end up with a capacitance of over 3000uF.

Now what would be the most appropriate porous insulator? Would paper towel withstand 200V? Does anyone know?

You are obviously misunderstanding the design principles of electrolytic capacitors.

The paper layer between Al foils is a separator, not an insulator. The voltage strength is achieved by the oxide layer thickness, not the paper.

The relative high specific capacitance achieved in your experiment suggests a thin oxide layer and respective low voltage strength.

I would guess that salt solution isn't the best dielectric anyway, it is rather conductive! Al oxide will almost certainly react with it disastrously, especially with a voltage applied across it.

I'm curious how you measured 5-6uF, what did you use to find that figure?

Brian.

That's what my multimeter said - about 6uF for 25cm2 of Al tape in MgSO4 solution.
Got any other suggestions as to how one could make a good electrolytic cap?

You would have to perform micro-etching on the aluminum foil.

All other things remaining constant, the capacitance increases with the plate's surface area.
If you do not want to end up with squares meters of foil, then you have to micro etch the foil. All the pits and craters vastly increase the actual surface area, multiplying the effective plate area many times that of the actual foil area.
The problem lies because then you now have to grow a very thin oxide layer that follows those intricate contours.
A poor oxide layer growth will result in either excessive leakage current or failure.

I once visited a Matsushita Electric (now Panasonic) capacitor plant in Japan. The section where the foil was etched and oxidized was completely off-limits to visitors, as this is where all the intellectual property content lies.

Another area of very high intellectual property is the exact electrolyte formula. The basic chemical elements may be found in good electrochemical books, but those specific chemical enhancers, along with the precise amounts required, are closely guarded industrial secrets.
This is not folklore or an urban legend. Dell Computer actually lost 300 million dollars on capacitors with defective electrolyte:

"A scientist steals a secret formula for an electrical product from his Japanese employer and takes it to China. Then it is stolen again later that year, the scientist's staff defected to Taiwan, taking with them a copy of the electrolyte formula so they could set up their own company.
Taiwan supplies 30 per cent of the world's electrolytic capacitors and most of the big PC manufacturers get their machines assembled in Taiwan. But the defectors mis-copied the formula. After a few hours of operation, the electrolyte would leak hydrogen gas, before bursting the metal body of the capacitor. The electrolyte would then leak its brownish filling and could cause a fire.... and thousands, perhaps millions, of computers and electrical goods in the West begin to burn out or explode."

"It sounds like the plot of a thriller, but it's reality. Thousands of computers have failed and nobody is sure how many more products might go wrong because their capacitors - essential components to control the power supply - were made with faulty materials."