I have been working on a piece of redwood, and I have run into a problem. As I sand it down with my orbital sander starting with and moving through to 220 I start to get dips. The grain seems to stand up as I sand and get a lot of dips. I have only encountered this once before when I was sanding some ash and could not get the grain to lay down. Any hints on how to get the grain to stay even with the rest of the wood. I have never had this situation with any other wood that I have worked with.

Welcome aboard canoeman, hope you stick around and get answers to any & all questions you might have.

I have not experienced this, but I haven't worked with redwood.  I have ash though.

My first thought is that you may be applying downward force to the sander?  Maybe using a worn sanding disc and compensating by adding pressure to the sander might cause your issues.

possibly, a 1/2 sheet jitter bug type sander would solve the problem. 

I have not worked with Redwood but have done some things with pine and cedar. The contrast between the softer and harder parts of the grain are probably what is causing this to happen.

 

Here's an article from Fine Woodworking that may be helpful-

https://www.finewoodworking.com/forum/problem-sanding-redwood-evenly

 

 

First, welcome to the Patriot Woodworker @canoeman53. Glad you found us and look forward to you becoming a regular. Great group here always willing to share, help and just chat.

 

Lew likely has hit on the probable cause(s) in this particular case combined with "feed+speed" rate. Perhaps these videos might give a few nuggets of help. Another option in your particular case this time might  be using hand scrapers if you have them for final surface leveling and finishing. Please keep this thread active as a learning aid for current and future readers.

 

All is good but about 10:15 in might be most helpful.

 

Not sure if this applies in your case, but good general  overall ROS practices.

 

 

In the case of raised grain using a sealer will help considerably. You can use a thinned coat of the final finish, sanding sealer or shellac. This will stiffen the grain and not interfere with final finish.

 

Scrapers are good but you so have to be careful to scrape downhill on the grain. Redwood is a very open porous wood and you will probably have to use a grain filler to get a dead smooth surface.

I have worked with redwood much for lots of years-----sounds like you are oversanding----removing soft grain,leaving hard grain---never gone beyond 150---sand lightly, if standing fibers are a problem--use a lightly damp cloth and follow with light hand sand. Dewaxed Zinsser does wonders on redwood.

I did away with my oribital sanders years ago and only use wide long flat sanders. They were originally made for the metal body works and on soft wood where you run into soft and hard often in wood close together like red wood this will show up more so than say maple or cherry or ash as they are mostly all the same hardness all across a board so this problem does not show up so bad.   

My orbital was great for 15-20 years but the pad is starting to crumble apart like the heels on old Red Wings. It is not applying even pressure like I remember. The orbital before that, a 2-speed, was even better, but it disappeared after only a decade of ownership, so I'm not sure how it fares.

 

A guy I worked for 1999 - 2001 built custom tables; he had his guy hand sand with a small block each table up through every grit. It took one month to hand-sand the table that way.

 

I've thought about going with the DuraBlock and roll paper but still have not made the investment. I make sanding blocks to suit each job and try to limit use of the electric sander to knocking down particularly high areas. Before going to hand block, then a hand scraper.

 

The trick to sanding redwood or any other softwood is to flatten it out with a new sheet of 80 or 100 then hit it very briefly with the subsequent grits. The issue is that the redwood has a wide gradient between the soft and harder areas and the sandpaper is removing them at different rates. I stop at 150-180 for softwoods. If I’m looking for a smoother look I achieve it with the finish. 
Paul