I'm trying to help a non-handy person fix a big gouge in a piece of exterior trim (car vs corner of house). Normally I would cut out a rectangle section with a router, stick in a dutchman and plane it flat.

There are all kinds of videos of people just filling it with Bondo or WoodEpox. Anyone have any experience with these? Do they hold up?

I've used Bondo on wood lots of times on commercial construction projects.  I've used it under Formica to get a flat, smooth surface.  I've used it on intricate trim details where something was curved or round and making round trim was impossible on site.  I've used it like spackle to repair gouges and voids in historical trim that we wanted to save instead of replace. And I've used it for exactly what you're doing.  Patch a spot and "spackle" the gaps and voids with bondo.

Cover with paint and it will last forever.  Two tips: (1)  Work in shallow layers of about 1/4" thick.  You can build up layers but if you try to put it on too thick in one layer, it can pop out when you're shaping it. and (2) Add too little instead of too much.  It's much, much easier to add a little as another layer than to have to sand some off.

Yup I've used body filler just not the bondo brand  personally. Get clear hardner because sometimes the red or blue can bleach through top coats especially white paint 

Bondo sticks to everything. I've used on exterior wooden garage doors.

We had some wood rot that the painters fixed with bondo, the results were amazing.  Can't tell you how long it lasts as it was done just before we sold our home but the painters said they do it all the time and never have an issue with it.

Thanks all - I guess I'll take a stab at it.

1 hour ago, Handyman said:

Thanks all - I guess I'll take a stab at it.

Pro tip: if the wood is dented( compressed. Take a nail and pepper the area. Helps bondo bond.

We are kinda doing suggestions by Braille here. Small stuff bondo, big stuff replace.

3 hours ago, Zeke said:

Pro tip: if the wood is dented( compressed. Take a nail and pepper the area. Helps bondo bond.

We are kinda doing suggestions by Braille here. Small stuff bondo, big stuff replace.

Or 36 grit sandpaper. The scratches give it something to grip on to. 

33 minutes ago, fishnut said:

Or 36 grit sandpaper. The scratches give it something to grip on to. 

Ya. Nail holes more for casual swing and a miss.... but I never miss