A friend of mine has a teenage son trying to restore a car.
He asked how to smooth out the fender flares which in the picture appear to be riveted into place.

Not having worked any fiberglass on cars, I didn't want to give him wrong advice. So please provide some detailed steps that I can relay to him.

Not familiar with the vehicle, or the flares. But if it's fiberglass to fiberglass, it's not hard to do. If it's any dissimilar materials, there will be problems.
'Glass to 'glass: Working in small sections, maybe 12 in. at a time, remove the rivets, and grind down the flare and the fender, going deep at the seam, gradually feathering out , the farther out you go (this is similar to blending paint). Then use cloth for the first couple layers, starting with pieces narrow, but wide enough to straddle the 2 parts. First layer I usually make around 2 in. wide, second 3-4. Then go wider, using matte instead of cloth (cloth will witness up into the finish, if too close to the surface. Until you've reached the edges of your grinding.
When cured, proceed to other sections. overlapping each start/stop area. If doing a real good job, do the backside in a similar manner. Make sure the meeting area of the 2 pieces on the back, is ground deep enough to reach the "new" fiberglass on the outer surface.
When cured, grind down, and use filler to complete the repair. Fi deep spots remain, after grinding, either use more matte, to fill, or if in a hurry, use fiberglass strand filler to fill the deep areas. I use regular filler to finish.
Dissimilar materials you may be able to get away with using a structural adhesive to bond the 2 pieces together, and finish with filler, but in many cases the different expansion/contraction rates, and flexing, will eventually show the parting lines, or even crack.

Nailed it!

Thanks I saved that information for myself as well.

I just asked the guy and the car is a 1972 Porsche 914. So the fender is metal and the flares fiberglass.

Is there a recommended panel bond adhesive for this?

Oh, god, been down this road on the 914 and the 911 which is even worse. You just cannot bond these fiberglass fenders on to that metal and smooth them over. It might take a month, 6 months, or a year but the vibrations and harshness of these cars are going to unzip them. No panel bond that I am aware of will prevent these things from coming back off. On the 914 your best hope is to use decorative nuts/bolts or a more decorative rivet and do some type of gasket between the surfaces. A few years ago I was trying to help a guy on here doing the wide body rear fenders and was grafting aftermarket flares to the 911 metal. I told him to loose the flares and just source the correct all sheet metal ones (they would have cost a fortune at the time) but nope he finished them up, painted it, and was so disgusted with the micro cracking that he quickly sold it..... Porsche cars put a lot of harmonic vibrations into their sheet metal from those motors, especially the old air cooled ones.

I asked this question on the forbidden site as well.
Most felt the panel bond adhesive would be the best choice.

As stated earlier, I have no real experience with this issue. I looked at the 3M 08116 which is basically made for this type of application.

My thoughts would be to prep the metal and fiberglass flares. Apply the panel bond adhesive and install the flares using rivets. The rivets would later be covered with the filler like Chopolds recommended.

The fillers, rivets, and metals will all heat and expand at different rates. My guess would be the rivets popping up first, followed by micro cracking. A huuuggge problem with the 914 is that mid engine location. Heat just boils up in those fender cavities from those motors. I've used an infra-red pyrometer in those fender areas and got readings of 130 to 150 F. Stuff heats up, cools down, heats up, and on infinitum..... = fail.....
Panel bond manufacturers are aware of heating problems....
https://www.chemical-concepts.com/summe ... adhesives/

Darrel is right, just think of how many cars you've seen with fiberglass scoops bonded to metal hoods, and how they failed. Joining any dissimilar materials is a crapshoot, with a 95% failure rate!
I would step away from the project... especially seeing how fussy Porsche owners can be!

I agree wholeheartedly. If it were me, I would refuse such a project, however it is a teenager who has some fancy visions for his car.

I have forwarded all of your comments to them, so we'll see what they decide.

With the "young" that's all you can do. Show em', tell em', and let em' do what they will do. Just saw a guy that "bedlined" his 6 foot truck bed with 12 cans of Rustoleum 1k bedliner. I tried to get him to do the math with me. A 15 ounce can putting down 13% solids uhhhh... that work out to be just under 2 ounces X 12 sooooo.... you put down like 24 ounces of protection over that whole bed.... You paid $84 plus tax for the cans. So you could have bought 4 liters or Raptor for like $129 and ended up with 8+ pounds of protection..... Hmmmm..... he just shook his head and said he was still happy with the results.....