Let’s make this a two-part question:
While I'd guess the answer to #1 is “buy such-and-such product and use it” I’m currently living in Colombia and will not have access to any special treating products.
I have a rain jacket with an Event® membrane.
You indicate your rain jacket is made with eVENT fabric.
If that is the case there are NO nature-gatherable substance that could be applied to your shell that would NOT result in losing breathability and probably permanently destroying water resistance characteristics of the eVENT garment.
eVENT membrane is highly sensitive to dirt, oils and other substances.
Unfortunately, chemical cleaners such as Granger, Nikwax, McNett, Penguin, are the method for resolving DWR issues with an eVENT garment.
Best recommendations if you cannot acquire those is to use warm soapless water and lightly scrub the garment and air dry it. This will not resolve DWR loss, but it will help improve the eVENT membrane at what level of water resistance it can still perform.
When jackets get dirty and tired, they no longer 'bead' rainwater and will 'wet out' easier - This is when the outer layer becomes saturated with water.
They should still be waterproof, because the inner membrane isn't affected by this. It's 100% waterproof, regardless.
However, they won't feel waterproof, because the breathability relies on the water-prepellent properties of the outer layer, which is now saturated and so won't pass as much air. So, any perspiration or moisture from your body has nowhere to go, and you'll feel wet - even though the water's not getting in from the outside.
Anyhow, have you read eVent's care instructions?
Always follow the garment manufacturer’s care and use instructions. Below are general considerations for maintaining garments with eVent fabrics. Avoid heavy soiling, which could affect membrane performance temporarily. Thorough washing will generally maintain and restore product performance. Regular washing of your eVent garment will enhance product performance. Your garment may be machine washed in warm water (110F / 40C) with liquid detergent, and we recommend a second rinse to remove all traces of detergent from the fabric. We recommend you drip-dry your garment. Remember also to refer to the garment manufacturer’s guidelines. Do not use fabric softeners or bleach. Never dry-clean your garment. We also recommend that you avoid ironing the garment, but if you do, use the steam setting on no higher temperature than “warm”. At some temperatures, the heat from the iron can have a rejuvenating effect on the DWR finish but there is a greater potential for fabric damage if the iron is too hot.
So, summary:
I've heard that leaving the jacket in a car on a hot day, or carefully using a hairdryer on it will improve the DWR without the risks involved in ironing.
Primative waterproofing is accomplished by impregnating s material with an oil or wax or blend of each. Rubber also works well and since you are in Columbia, you can probably make your own from a rubber tree.
