Scaling Images for maximum quality for Trade shows. Progressive Vs all at once

Former Photoshop engineer Chris Cox always said that incremental resampling had no benefit or purpose. It would only amplify artifacts.

Upsampled jpegs usually look horrible because of the jpeg compression artifacts. They do not upsample well, to put it mildly. For that reason, I'd try the new neural filter to remove jpeg artifacts first. I've done some lesting, and it seems surprisingly effective.

Aside from that, upsample only as much as absolutely necessary to avoid visible pixels. Upsampling will never improve anything, and the image will always look best without any resampling at all - even when reproduced at large sizes. Consider that a traditional computer monitor is around 90-100 ppi. Do you really need more than that?

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1. Use the Image Scale Tool. Uncheck Resample image and ajust the dimnetsion to achieve the scale so the final is 100% sized at 150DPI.

2. Progressively scale the image in smaller increments. So 200% with the same process as above and then repeat it until you acthive the final 100% size at 150dpi.

OR

3. Is there a quality difference or benefit to progressiely scaling an image?

Is there a 3rd party app or tool or photoshop plug in to achieve the quality better than Image scaling?"

First I'll point out that this is going to be risky with a Jpeg since the original compression will have created artefacts that may well become visible as size increases. 

I've used progressive step by step upsampling [on advice from Pixl Aps, Thomas there is a serious Photoshop expert) with a client who make VERY big prints and doesn’t want to see pixels when viewed very close. 

Youd need to make a test [first save the Jpeg as a TIFF so no more resampling ensues], it may be that today's more sophisticated Photoshop (or the app you mentioned) can do a great job without this being needed. It certainly was needed in the days of the original low rez digital cameras and it worked quite well..

BUT, I'll mention again that starting with a Jpeg, you're on a sticky wicket. My friend the Photoshop guru Martin Evening wrote that making a raw into a Jpeg potentially discards up to 83% of the image info. 

FIRST take your 18 x 28" at 300 dpi jpeg and with resampling off and set the rez to 150ppi, that’s some free expansion of printed size. 

I hope this helps
neil barstow, colourmanagement net :: adobe forum volunteer:: co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management

Way back in the day, upsampling in steps could sometimes give better results than all at once. This is no longer the case, just use the tools in Photoshop or a third-party plug-in that specializes in upsampling. Some developers have AI/machine learning softwatre that can improve quality.

If I were that worried, I'd try a couple of different methods and then print partial samples (say, 10% of the banner) at various ppi to see what looked good in actual use.

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1. Use the Image Scale Tool. Uncheck Resample image and ajust the dimnetsion to achieve the scale so the final is 100% sized at 150DPI.

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Does the original image already have 150ppi effective resolution at the intended size without resampling anyway? 

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3. Is there a quality difference or benefit to progressiely scaling an image?

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How could that bring benefits compared to upsampling in one step? More artefacts would not seem a benefit.  

Which Resample Methods have you tried? 

What is the effective resolution your print provider requests? 

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Is there a 3rd party app or tool or photoshop plug in to achieve the quality better than Image scaling?

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You could try topazlabs’ Gigapixel AI. 

And as for terminology: 

• No Image Scale Tool in Photoshop. Did you mean Image > Image Size? 

• Digital images’ resolution is given in ppi, not dpi.