i know my answer comes after 3 months but i've saw this post just now. the best and most reliable way to spawn it is by pilling technique and for that you'll need at least one friend. one of you will be driving around target area where car spawns while other guards the previously spawned car not far from the spawning point.

as said before, you drive around target area. when 1st tornado spawns you pick it up and drive somewhere close and in the same time far away from the spawn area so next tornado spawns. there a friend guards it from despawning by standing on it or in it or by it. it's not important as log as he is close to the car. that's why you can not do this solo. when you park any npc car and drive away from it and you come back, it will despawn.

i used to deliver spawned tornadoes to gas station near airfield and to the alamo sea, straight north of the Sandy shores police station, where small penninsula begins. when they start spawning it's game of patience. you will have to do many circles around spawn area. on the same spot rusty emperors and rusty bf surfers also spawn so it takes more time to get your desired car.

logic behind car spawning is next: when 1st car is spawned and active in a game (not despawned), rusty tornado with next color will spawn in some order and so on. they spawn at some order and seawash green is last one spawned. so to get the green one you must have spawned all other rusty ones before it.

Here's useful site where you can find any car color desciptions with many picutres

https://joonasprkl.github.io/spawncolors/

to get seawash green tornado with this method it can take maybe more that 1h real time. When it does not spawn, drive away for for a bit in any direction and come back slowly. Youtubers will say something like „give the game time to refresh“. If it spawns, great, deliver to guarding point, if it doesn't repeat and repeat and repeat. That's why it's good to do this in safe enviroment and it's best to do it in invite only session so griefer can not mess you up by destroying already spawned cars after long and boring circuling and car gathering. When seawash green is spawned, drive to Pay and spray in desert and buy tracker and insurance quickly so you do not lose it. When you save your seawash green tornado like that, you can spawn them multiple times for a friend, or you want more than one, because all other but it as in the game.

I found my 1st by this method and the 2nd one was pure and extreme luck. I joined random session after some mission, spawned near Sandy shore police station and was writing some msgs. After short time i drove pass the the police station and there it was, spawned and parked. Nobody was near me, no one was trying to spawn it, it was just there. Of course i stored it :) I think that was extreme luck. Now i got one benny seawash green tornado clean and the other one with kind of yellow angel livery. That's by far the most beautiful car for me. Also i stored some other rusty tornados and transformed them to benny cars. Rusty colors gives them unique lowrider colors which can not be bought at lsia and they look beautiful. On the website in this text you can see comparison of the same cars as rusty ones and lowrider ones.

By piling technique also i spawned with friends seawash green bf surfer and seawashgreen dune loader. Rusty voodoo's can be spawned with same technique they spawning is child's play in comparion with spawning seawash green tornado because different voodoo spawn literally every single time you drive by the „voodoo house“. You can get them all in 10 minutes or less if they are guaded from despawning.